You can buy it piece by piece. You can modify the receiver, bolt, trigger, mag release, sights, and stock. Lots of barrels to choose. It offers areas that fairly technical yet easy enough to gain a understanding of how it all works together. What it takes to hotrod it.
See if there are any stupid "gun buy back" events near you. Go there, set up a table on the sidewalk outside, and offer $10 more than the folks running the buyback for each gun. Not only will you get plenty of guns in varying conditions to practice on, you might save something valuable or historically significant.