I built 3 "home-made" steel frames a while back, and raced quite well on one of them (the others were for my brother & a friend). I made my own brazing jig out of flat stock aluminum plate. I also bought torches and rented oxy-acetylene tanks. I bought tubing sets and all the lugs and braze-ons and had fun. It's a lot of work though and you should have some metal working and brazing/welding background.
Aluminum ?? That would be a lot more difficult because you need to heat-treat or temper it after welding. (Last I knew)