Well I would say, as long as you're not doing like a hard tail conversion or something and are sticking to pretty much non-structural components (and you have a lot of practice of course), any decent welder will work. I don't know about minimum amperage, I would guess it varies from what type of welder your using, how thick the metal is, and how thick your wire/rod is.
I managed to weld my frame hoop on with a craftsman arc welder with 1/16th harbor freight rods on the maximum amp setting (90). The first couple of times it didn't stick. This is because I would beat the shit out of it with a hammer to make sure it was solid. Eventually, I got it hot enough that it finally penetrated. That metal was thick. If you're just welding tabs on, I think you could do the same with my welder with less issue. I managed to weld 22 gauge sheet metal too on a lower amp setting (it goes down to 30 or 35). The welder cost 150, more rods were 10 and it came with a few rods, a chipper, and a ghetto face shield you hold. It runs on 110 volts too.
But if you want something that is easier to use... (and have the money more importantly) by all means go for the mig. I wish I had one, but my arc works.