I've owned an HK sear gun for years (MP5SD) and have never really shot it much.  In fact, I'm not particularly knowledgeable about the HK platform in general as I've been an AR shooter most of my life.  I don't particularly like the SD gun, and have an interest in obtaining a quality new 9mm host for my sear and trigger pack.  I have no idea where to start looking, and suggestions would be appreciated.  Ideally, I'd like the capability to use a detachable suppressor,  ability to use modern sights (I.E. picatinny rail), and lights etc.  Are there quality clones that will readily accept my sear pack, or do I need to find a good HK builder to make a conversion?

HK SP5 can become sear ready for about $500.00. It does not have a rail but one can be attached.

PTR 9ct is sear ready but for FA may require work to be reliable

All below are very well known in the roller lock community and can't go wrong with either. All can be googled for com's

Parabellum Combat Systems will build you an MP5 type clone that will look and run OTB. I believe he does/can weld a rail on top.

Michael's Machine does customs.

TSC Machines does customs.

Dakota Tactical does customs

What caliber are you looking at?

9, 40, 10mm, 556, 308, x39

Lots of options out there.

What don’t you like about the SD? It’s a pain to clean but other than that it’s got a lot going for it especially with a sear.

My mind may have went the direction yours did until I re-read his post, he specified 9mm. I was thinking a 53 or 33k. That way he would have another caliber of choice. Why he doesnt care for the SD, I'd like to know that myself. Like you stated, other than the cleaning, what isn't there to like being able to shoot anything out of a barrel that length and not worry about the sound? If he is hell bent on 9mm, maybe an MP5K?

edit: damn dyslexia..always transposing letters...

There are two reasons I don't like my current MP5SD.  

1st, as some have suggested, it's a pain in the ass to clean.  I would like to shoot the hell out of it, and not have to clean a suppressor.  Yes, it is very quiet, but many times I don't care about suppression when recreation shooting.  I'd much rather have the choice to shoot suppressed or un-suppressed, and have a serviceable suppressor when cleaning.

2nd, it would be nice to update with modern optics, lights, etc.  There are no rails on my current host.

When comparing my full auto MP5SD to my semiauto Sig MPX, I sure wish I could own the full auto version of the MPX.  Just saying!

I have an HK91, which I bought thinking someday I might convert it to accept the sear.  Frankly, if I was going to go rifle caliber, I'd want a belt fed which would be more expensive to shoot.  My current shooting options are limited to sub guns, so it would be hard to shoot anything other than sub calibers anyway.  So 9mm makes sense to me.

So assuming I bought a new SP5, who are the reputable guys to do a good conversion, and any idea what the costs would be?

There are only 2 people I know that have done sear conversions. Michael's Machine and TSC. Both achieve in a different manor. The cost is about the same IIRC. As I mentioned above, TSC's is around 500-550? I would call and chat with each and let your conscious be your guide.

Edit: A popular thing people are doing with SP5's is remarking to MP5 or MP5 N. You may want to chat about that as well with them if it's something you might consider.

Wow. What a fantastic question to ponder.

If I had a sear, what host would I want? Well, all of them...

But if I had to choose, it would probably be the plain-jane MP5 with fixed A2 stock. The PTR 9CT is as close to sear ready as they come, and it already has a top rail welded on (which you stated you prefer). They also come with the M-LOK railed forearm. I think that's just what you are looking for.

Personally, if I invested the money in a sear I would want to do more with it, and would eventually end up with an MP5K-PDW and probably a 5.56 version as well (HK 53/33?). But honestly, it is the MP5SD that makes my heart skip a beat! But I might consider a "clone" that uses a standard barrel with a detachable can, but just made to LOOK like an SD.