The only part that could be restricted would be the lower receiver; the part with the serial number. That, and high capacity magazines. The rest is just parts which there is very likely to be no restriction on.
Last time the administration went crazy on "black guns" I bought complete lower receivers and put 'em away in the safe until I could afford to complete my rifles. You'll have to buy the lower receivers, either stripped or complete, from someone with a FFL. If you are the least bit tight on cash, buying a receiver or two and a handful of magazines is a solid strategy.
I can't comment on the assembly part of your question as I bought complete lowers, and mated them with pre-assembled tops as the checkbook would allow. There are a million good resources for this on the web; If you're the least bit mechanically inclined from what I hear it's not a big deal. From what I know, getting the barrel headspaced properly is critical but I'd think any good gunsmith could help you out there.
....black guns are cool....
Have fun - B.B.S.