As was mentioned, there are several conversions of HK firearms.
When you buy a full auto HK, you get it in one of four flavors:
"least" desirable (for an MP-5, that means in the $7-8k range) is a "clip on" Registered Receiver gun. The manufacturer that converted the gun took a regular semi-auto HK 94, registered the receiver, then modified a full auto lower to fit the gun. You cannot legally separate the lower from the receiver--kind of like having a non-registered DIAS in a Registered Receiver M-16. Take the DIAS out, you have an "unregistered conversion part".
Next is a Fleming trigger pack. A little different from a sear (not sure how exactly), but it is interchangeable to other HKs.
Somewhat more desirable is a Registered HK Sear. Most guns you see are sear guns. A loose sear runs $4500-5000. An MP-5 Sear gun is in the $8k range. The sear can be swapped around to different guns. Lots of sear manufacturers--S & H, Ciener, Fleming, Qualified, HTA, etc.
Most Desireable is the "Push Pin" Registered Receiver gun. The guy that converted them registered the reciever, and modified it to be identical to a real full auto receiver--interchangeable lowers from factory MP-5s will work fine. These run at $9k. Billistics is the best known of these.
Of course, there are the rumored REAL FACTORY HK MP-5s out there--supposedly there are a few transferables in the country. Gotta figure on $20k minimum for one as a collectable.