I've looked into doing the same. I've found tutorials online about converting the bolt from open-bolt to closed bolt, neutering the fire control group to semi only, and grafting a front trunnion section to an 80% receiver. The bolt conversion looks like the most daunting aspect, but it's not impossible, just a lengthy process. Unless you're tooled up to do all the machining, I'd just buy a semi-auto closed bolt. The receiver-trunnion grafting will require some decent welding skills.
When it comes to building guns from parts kits, it's gonna cost about the same as purchasing one that was manufactured by someone like Century or Vector. But the knowledge gained by solving all sorts of problems during the build is invaluable. If you're up for the challenge and sense of accomplishment, go for it. Careful though, this gun building stuff can get addicting. I plan to build an UZI one day, but first I need to finish a CZ-26, Sterling, and SIG STGW-57 (this one will probably take forever).