clayman, welcome to UZI Talk.
Building a semi kit yourself is not as easy/cost effective as some people think. You can't just buy a receiver and an SMG parts kit and be ready to go. There are two problems. #1, the parts kit is from a full auto gun and you'll need some semi parts to build the gun properly. #2, your parts kit is all imported parts and wouldn't give you the US parts count you need to legally build it. I'm not a kit builder so maybe someone else here can give you specific info but just ask a lot of questions before you proceed.
The IMI receivers are probably a better option for building a kit. At least they will be in spec, which isn't always true with the GI receivers. The receivers marked "N H M C" are IMI manufactured receivers. They were surplus semi receivers that IMI had after the 1989 ban prevented them from importing any more guns to the US. D&D sells the receivers. Vector uses them to build their post-ban semi guns. I'm not sure if Vector sells just the receivers but you sould askl them.