It might be worth it for a sear gun, but for a semi or SBR it would be a waste of money IMHO.
Assuming genuine HK parts: (I'm not going to give how to project for SWs, since I'm morally opposed)
HK94 $3000-3300
Post sample MP5/.40 to demill for the parts $1500
This would be the way to go, since finding the parts separately wouldn't be any cheaper.
Now the tricky part, finding a competant gunsmith. I wouldn't trust just anyone who took an HK armourers course. This is major work for someone like Terry Dyer or Murray Urbach.
Expect to pay $1500-2000 (rough estimate) for the services.
$6000-6800 total for a very unique semi auto, + a bit more if you aren't going to SBR it, since you'd need a fake can or other bbl extention permanently attached.
Yeah, it would be cool and pretty unique (I've only seen a couple floating around), and if you had a registered sear, than it might be worth it. But for a semi, too much $$$ for my tastes.
Preban .40 mags are hard to come by, expect to pay $125-150 each.