Building a semi-auto MP5 (called an HK-94) is not as simple as an AR-15 build. The concepts are the same but this isn't someone can do in their garage on a Saturday afternoon. On HK guns, the reciever (think upper in AR-15 world) is the controlled part of the gun, not the lower like in an AR-15. That's why they sell the trigger pack with the parts kit without going through an FFL. Now for the problems:
For one thing, an MP5 parts kit will come with an 8" barrel which is, of course, illegal for us lowly peons to own without getting permission from the ATF.
The second problem is that you're going to have to find the receiver to put all the parts into. These aren't very easy to find, currently there is only one company making them here in the US and as far as I know they aren't selling just the receiver. The company is Special Weapons, you might check the forums at www.hk-54.com/board and check under the Special Weapons section, that is where Todd Bailey, the owner, will announce when he's selling receivers.
The third problem, should be manage to get an MP5 / HK-94 receiver is that you're going to have to put all the parts into the receiver. The biggest of the hurdles here is pressing the barrel into the trunion which requires either a 5 ton press and a lot of know how. After getting the thing assembled, it would then need to be refinished.
Basically, you have a couple of options to get what you're wanting. First, you could buy a US made clone of the MP5 / HK-94 made by Bobcat Weapons from Atlantic Firearms. It's called the BW-5FS and it should run you around $1500.
Link to BW-5 on Atlantic FirearmsThe second option is to buy an honest to goodness HK-94 for around $2700-ish. You might check places like www.gunbroker.com www.sturmgewehr.com or www.gunsamerica.com for listings.
HK-94 for sale on GunbrokerFinally, you could do what you're talking about, buy the parts kit, get the receiver, apply for an short barreled rifle permit from the ATF, and send everything off to a gunsmith like Jayson at Investment Grade Firearms to have it all assembled into a kick-ass MP-5 copy. This would probably cost you around $3000 total, depending on how much you pay for the parts kit. www.investmentgradefirearms.com
Hope this helps.