You can't go wrong with a Miller. The major differences you'll notice between a cheap welder and an expensive welder are when starting an arc and duty cycle. I use the Miller XMT 304 welders a lot, and they do a full power start. You just place the electrode near the work, and it explodes into arc. None of this striking it like a match. Duty Cycle is the amount of time you can weld at a certain current before the welder has to shut down and cool off. I generally run the XMT welders at 135 amps and have welded for hours straight pausing only to change electrodes. The machine didn't even get warm. Then, I've used cheap mig welders which cycle off after about 2 minutes of welding near their lowest power setting.

If you're going to be boxing a frame, get a welder that can run at about 80 amps or so for a good period of time.