Agreed -- you ***really*** want to radius the edges, because the weld will crack if you just put a bead on a 90* corner.
You can easily whip up a set of T-Dollies for this job. They're designed so the stake rests on the cross-bar of your vise -- make sure that the stake is long enough so the round dolly part sticks up 3~4 inches above the vise jaws.
Use the T-Dolly to form the half-round-over on each side of the edge ( the intersection of 2 planes). Corners (intersection of 3 planes) are a bit tricky, but a little practice and you'll get the hang of it -- basically, leave the corners long, and file them back to about 45* after you've rounded the corner over.
I suggest that you fit everything first, just taping the pieces together so you can weld it all at once, preferably with back-gassing.
Also, I strongly suggest investing in some aluminum files - you'll never know how you got on without them if you do much aluminum fab. They cut fast and don't clog. You'll like'em, I promise.
Regarding your bends, make sure that you use a generous radius -- aluminum doesn't like to be bent sharp or crisp -- it will break along a sharp bend. The minimum recommneded radius depends on the thickness of the material, but a wider radius is always better b/c it won't crack as easily. Again, you have to build your corners carefully -- they'll be the meeting of 3 radiused edges.