I got an AP5-P as my first step into the MP5 world. My question is what spare parts should I get? I am running it suppressed with an Obsidian 9, and a brace.  Any other upgrade recommendations or must have accessories that you would recommend? I wasn't overly impressed with the stock trigger but am on the fence about a binary trigger. It's mainly a range toy that I will dress up as a home defense rifle.
To that end I ordered a Magpul sl handguard... If anyone knows of a trick to get that installed it would be much appreciated. (I have given up on hammering it in and will break out the Dremel this weekend)

If you have to hammer on the handguard, you might want to re-read the instructions.

As for spare parts: get an 80 degree locking piece of you want to shoot it suppressed a whole lot.

Also, get some spare extractor springs while you are at it. Those typically wear out faster than other parts. Stock up on good mags too. The MKE and HK mags are all I use and can recommend.

I ordered a couple of spare MKE parts kits from Atlantic.  It's also recommended to use real HK extractor springs.  Although I have MKE mags, I just spend the extra $15-$20 or so and stick with HK factory.  Lanbo's usually has good prices on Factory mags with a $10 flat rate shipping.  I have a couple of Binary triggers in my MKE's.  The semi mode is actually a pretty good trigger.  Heckfire is another option.  There's also a Poors MP5SD thread in the HK section if you want to go that route.  I got my Form 4 R9 approved yesterday and put this together.  Still waiting on my Omega which is the can I intended for this build.  This is an MKE with Binary.  All you need is the handguard retainer & outer handguard for around $170 (see the Poors thread).  The mount is a $10 cheapo of Amazon and is actually pretty solid.  Just use locktite.

Rail Mount Adaptor - Scope Claw Mount Picatinny Rail Mount, Fits for MP5 G3 8 Slots(Aluminumn)

The handguard is on there, it's just not lining up perfectly to thread the post thru. I have heard about the 80  locking block for suppressed shooting. I haven't had any issues with the stock locking block but I also haven't shot it with the brace on yet. I am with you on good mags and have all HK and MKE mags. I learned my lesson years ago about cheap mags.

The stock trigger was a whole lot nicer than the Franklin Armory binary that I put on it.

I have heard that before. I don't want to spend a decent bit on a not so great trigger. If it's a good trigger that's one thing. Did your stock trigger get better with use?

I must have a unicorn.  My stock AP5 trigger is quite satisfactory.

The Franklin Armory binary is much worse.  If I had known, I wouldn't have bothered.

That's the exact opposite of my experience. The stock trigger was okay. The Franklin in semi is much better.

As far as spare parts, I bout a few MKE ejector springs but haven't needed them. It has fun flawless, both suppressed and unsuppressed. I never bothered changing locking piece.  

It shoots 147 gr subsonics almost exclusively now.

I put a Franklin binary trigger in mine as well and chose to just install it in a Navy lower, so I still have both.

Comparing apples to apples the Franklin has a better trigger pull in non binary mode.

I did not like the right side safety lever that came with the Franklin trigger and swapped the levers out with after market levers, where the right side lever does not interfere with trigger finger position.

When braces were targeted by the ATF, I registered mine as an SBR and just went ahead and put a stock on it.  

Some upgrades are worthwhile, like the current style H&K ejector, and the 80 degree locking piece on a suppressed "K" PDW.

If you really want to spend some money, the M.A.D. Max or R.C.M. upgrade bolts have a vastly superior extractor which uses a coil spring. The design of this extractor and its spring were "lifted" from a couple of H&K belt-fed LMG's so it is a proven, durable design. This eliminates the MP5 Achilles heel, the failure its OEM "bent wire" extractor spring.

I sbr'd mine. A range toy deserves a binary.  Shopped around and found a good price on a Franklin. 270 rounds the first range trip had me giggling all the way home.  1000 rounds through it with absolutely no jam, no stumbles, no hiccups....nothing.  Bang every time with S&Bellot 124g through 7 MKE and 3 HK mags.  I have extra HK extractor springs, but the gun is all MKE right now, sans verticle grip and folding stock.  Waiting on stamp for an Obsidian 9.

Love it.