We used a Lincoln MIG 255XT throughout high school. Damn nice machine, never had a problem with it, even with 120+ kids using it on a daily basis. Also had a Miller Millermatic 252, nice machine, but didn't get used quite as much.
Something to look at when buying a welder is duty cycle. The duty cycle is the amount of time in a 10 minute period that the welder can be used. Its rated in percentage, so a 60% duty cycle at 225 amps would allow you to weld for 6 minutes at 225 amps before letting it sit for 4 minutes, to avoid over heating and damaging the machine. 50% would allow you to weld 5 out of 10 minutes, 40% 4 out of 10 minutes and so on. As your amp setting goes down your duty cycle increase, so on the same machine rated at 60% at 225 amps, at 100 amps your duty cycle could be 90%-100%. So basically a high duty cycle is better. But be careful if you are buying an older machine, over time the duty cycle lowers, like how you lose horsepower in an engine over time.
For a MIG I like 80% or higher, for TIG or stick 50%-60% is fine.
Wow sounds like I know what I'm talking about, maybe I am actually learning something in school.
-Jester