To make a 48" long bend in mild steel you need to generate at least 14-17 tons. That's a lot for a manual press brake. Take a look at a used 4' Di-Acro press brake. I see alot of them at auctions. Note they usually run 240V
Here is a youtube of what I'm thinking would be good for your use:
http://www.youtube.com/watch?v=G3NjSco-Xgs
For bending just 90" bends you can use a v die but if there are return flanges you'll need to use a goose neck die. For the robots you build stick with 5052 H32 aluminum. Don't use 6061 T0-T6.
Another good little press brake is the Mitsubishi Electric BAll screw press brake. They are expensive but easy to setup and have a small foot print.
http://promo.mitsubishi-world.com/pr...f/electric.pdf