Check out Investment Grade Firearms. Jayson does great work and his site breaks down the service/price well. If you go the HK route, you don't need much in the way in parts. Get yourself a HK91 and a HK33/93 forearm. There might be a few other items, but thats about it. He can cut the barrel and front of the rifle down for you. He can also make it the correct SBR length or permanently attach a slightly longer flash hider to keep it at 16 inches and prevent you from having to SBR it. Hope that helps.