ok, buying a welder really depends on what you want to do with it. If you want to tig weld, you can't beat a miller. If you want to mig weld or stick weld, a lincoln will work great. For a budget shop, a miller syncrowave 250 will tig weld aluminum and steel. with a stinger, it will also stick weld on reverse and straight polarity. If you are into mig welding, don't go cheap. Buy a lincoln invertec 350 with a wire feeder. If you are interested in doing any welding beyond sheet steel, you can't use a cheap welder or your welds will be very unreliable. If you want one welder to do it all and have a little change, a miller dynasty with a wirefeeder will do mig, tig and stick, ac and dc.
Basically, here's how it breaks down:
Budget tig/stick: miller syncro 250, used
budget mig: millermatic 350, used
Better mig, lincoln invertec 350
best all in one, miller dynasty 350 with a wirefeeder.
Hope this helps.