Long story made short, there's a local company that makes very high quality HK clone products that is going to build me an MP5 of my choosing at cost. Hoo freaking rah. These guys make a full line of clones in semi and select fire but I am not cool enough for one of those.
Here's a little problem. Don't know jack about them. Going to be an SBR suppressed system that basically travels with me. It will be shot a LOT.
Either going to be a K with folding stock or standard mp5 with collapsing stock. Will most likely have optic and a light for home use. Suppressor is Omega 9k on a 3 lug. Those parameters being known:
1. Is the mp5k advantageous over the standard mp5 in any respects other than size and weight? Barrel length with supers gives SOME change, but with 9mm will I notice??
2. What are the weaknesses of the MP5 against its modern competitors? I am choosing this firearm over one of them in particular. Shouldnt be hard to figure out which one. ends in X.
3. Please don't hate me on this one. Not trying to start a debate. *sigh* are the key mod forearm offerings for the HKs gimmicky or a viable option for fitting the guns for your needs? I know there are purists here but I ask because I do not know.
4. What parts on the MP5s wear out first and at what typical round intervals? I've got 15k rounds set aside for break in and range time and I would like to have any and all spare parts I may or may not need.
I do appreciate all feedback. I am incredibly excited about finally getting one of these little beasts.

You'll get more velocity and range with an Mp5 over an Mp5k but I can't give you the specifics.  I will say that back in the day when they cost $450 I owned an HK94 and to this day I remember being impressed with the accuracy of that carbine at 100 yards, it blew me away.  I have a Zenith Z5P right now but I haven't had a chance to get it out and shoot it, it does have a SB Tactical brace on it but I don't really expect it to have any where near the accuracy as that 16" barrel HK94 or even an Mp5.  When it came time to make this purchase I chose the Z5P as I personally wanted to go the SBR route and figured I wanted an MP5k due to size, weight, etc.  I also opted for the Z5P over the POF because I wanted the threaded muzzle with the 3 lug which the POF that Atlantic had did not offer.

I had 5 9mm Carbines/Pistols.  I sold most because I didn't really need them all.  I sold the Colt 9mm AR, Sig MPX, and CZ Scorpion EVO.  I kept my IMI Uzi Model A and the Zenith Z5P.  The Uzi because its a friggin NIB Uzi Model A that I'll likely never own again and the Zenith because I'm going to SBR it.  The Scorpion I just couldn't get over all the plastic.  The MPX - if you read up on them there are a lot of feeding issues with them and I just didn't feel like having that expensive of a gun that I couldn't rely on.  It did feel and handle good - but not as good as the Zenith.  A MP5k is going to be much smaller and lighter than an MPX.  As for weakness the main would would be ease of mounting optics and accessories on the MPX as compared to the Mp5 series.  My Zenith came with a cheasy mount that I proceeded to apparently strip a screw on (damnit)...

I can't give you an answer on the keymod for an HK.  I'm more of a purist I guess.  I have Keymods on my AR's not my HK's.  I also can't tell you much on the parts wearing out as I've never shot an HK or clone enough for that to be an issue.   Usual suspects I would imagine, springs, firing pin, extractors, bolt rollers, etc.

I would recommend you go with a regular MP5A3 type first.  That's the workhorse of the MP5 line and shapes up very handy as an SBR.  

There are very easy ways to get a light onto the regular MP5, but the K model really wasn't designed for that.  It's a PDW/Close protection gun.  

They are both extremely fun as an SBR but the fullsize MP5 is the one I take out the most.  

As far as spare parts, you should have extractor springs and a spare roller retainer plate on hand.  That's about it.

Realistically, you're talking about a 4" difference. The MP5 with an A2 is about 21" long, with a B&T is about 19" long (collapsed). The K is like 14-16" long collapsed.

Muzzle velocity IMO won't matter. Subs will be subs with the can on.

On parts, you'll want spare extractor springs, ejector lever, maybe bolt head and firing pin/spring and recoil spring assembly.

I have a full size. It's compact enough that, while I thought I'd buy a K, I don't think I will.

Full size are easier to mount lights on than K's- at least more grip space after mounting.

I'm not an MP5 expert but I do dabble.
1. I don't think you'll notice a big difference in k vs standard within 9mm ranges.  Especially if you're shooting subs. The standard size is a pretty big gun for a 9mm thrower, approx. AR15 SBR size.  The K is significantly smaller than an AR15 SBR.  To me the K's size gives it a niche while the standard doesn't do anything better than an SBR AR15.  That being said most wont ever use their MP5 in anger, so go with what makes you happy.  The fullsize is a cool gun no doubt and 9mm at range is good hold / wind practice.
2. Mounting optics is slightly more of a chore on an MP5 but you can get some (Brethren) with rails already welded on making mounting a non issue
4. Check out Henderson Defense's high round count PCC thread for parts that break on his MP5's.

Can you point me to this? Looked all over and can't find it.

x2

The K is much more compact, I opted for the MP5. It was built as a pistol on new frame with an MP5 parts kit by Ghillebear. I also had him add the compliance parts, I then mailed the form and added the stock when I received the stock. More than happy with it. It now has a B&T stock.




 

I would start with the MP5. There isn't much you can't do to it, as far as accessorizing. Wide array of muzzle-mounted suppressors. You can get folding/collapsible stocks to make OAL fairly short.

For spares, I would reccomend:
Roller retainer pin
retainer plate/spring (depending on model)
Extractor
a few rifle extractor springs
ejector
ejector spring
ejector lever axle (more likely to be lost, than broken)

After you get the gun, and verify the bolt-gap is within spec, maybe get 1 size larger rollers, for a later date when it has a higher round-count down the barrel.

Adding that up, from HKParts, came to $175 and really should cover most any issues you will encounter with the gun.

Also, the MP5 runs both unsuppressed/suppressed  with the 100-degree locking piece, the MP5K, with stock and suppressor should have the #28 / 80-degree locking piece.

They ALL make me happy. That's the problem. I definitely see your point on barrel length being nearly a non-issue with 9mm here as well. Being that it will be shot suppressed more than unsuppressed, subsonic will be the usual food. At least that's what I'm stocked on. 147 fmj and 147 gold dot.
These guys do the welded top rail on their guns so that is no issue there. For me the K seems like a better option to stow away easily and get around with. I've fired their K model FA but again, mag dumps aren't the same as controlled pairs.
I appreciate the feedback so far on here. I'm anxious to learn more about this platform as time goes on.
I need to find that Henderson Defense thread. I get a kick out of reading the other ones I've found.

BLUE

I have spent a lot of time with an MP5A3 and little with a K , I would take the MP5A3 over the the K for numerous reason, the K just didn't do it for me. I unboxed the one I was issued brand new from Germany and  put thousands and thousands of rounds through it and never had a part fail. The guns just shot, semi, burst , FA any type of ammo it just ate it. The only thing I ever improved about it was the front sight, I put the XS 90 on it and loved it even more. Since that time I went to a Sig 556 SWAT, I like having the terminal ballistics of the 556 vs the 9mm but I still miss the MP5. its a weapon that is so iconic  and get into your soul. On a side note, we have a few 40 cal  MP5s made somewhere back in the early 90's that I would say have over 100k rounds through them. While I was at the range one day  I got one out ,they had not been fired in at least a year or two, ran 3 mag through it FA with out one hiccup . they have last round bolt hold open. I don't know why the 9mm never went to that set up. Just FYI these were all H&K MP5's not clones.

The keymod handguards are made by Dakota tactical, available at hkparts.net. DT is a good builder and what I've read about them seems positive. The old surefire HG and similar have issues clearing a can and 3lug adaptor.

I remember reading HendersonDefenses' review a while ago, I think MP5s were spoken about in the High Round Count AR/M4 thread here:
https://www.ar15.com/forums/t_3_118/677135_High_round_count_AR_M4_s__over_100_000_rounds__and_how_they_have_handled_on_our_range.html

IIRC extractors needed to be replaced.  Barrels lasted practically forever.

I had an MP5A3 (an S&H HK94 conversion) and was issued an HK MP5A3 at work as well as received various training over the years (back in the 1990's). The only part I ever had break (with some regularity) was the roller retainer plate. It's a wafer thin piece of cheap metal that holds the rollers together. I had my MP5A3 have them break as well as I had them break on the work MP5A3 and ever on training ones that I cleaned after shooting them. The gun continued to shoot with the broken plate, so it wasn't a catastrophic problem.

I ended up selling my MP5A3 and purchased an M16A1. While the Mp5A3 was a lot of fun shooting on full auto (especially with a sound suppressor), I was not that impressed with a 9mm submachinegun as far as an adequate self defense weapon. That's why I opted for a rifle caliber machinegun. It's a lot more satisfying and more effective for self defense purposes. If you're set on an MP5, then I'd recommend the A3 over the K. It's just a lot more controllable and accurate than the K and a lot easier to mount a weapon light. I had an HK SP89 (semi-auto version of the MP5K)and I wasn't impressed when compared with my A3 (not just because of the difference between SP89 semi-auto and MP5A3 full auto). The A3 just handled better ergonomically and was more accurate. And for some unknown reason the SP89 didn't like MP5 30rd magazines. It was brand new and wouldn't reliably feed rounds from my OEM HK MP5 30rd magazines. It would only feed reliably from the HK supplied 15rd magazine that came with it in the box.

I have both and just bought another mp5 full size.  The k is awesome but I love the full size!!

I have the K and now the full size. Both are fun to shoot.