Most TIG welders can also stick weld so you're buying two machines in one box. Stick welding is also much better when welding dirty, rusty or painted metal; as long as it's thick enough.
IMHO a used older welder is better than some of the newer machines with electronic controls. They rarely fail and if they do fail it doesn't require a service tech and a pile of $$$ to fix. I have a MillerMatic 35, built in the early '70's. Paid $233 for a clunker trade-in machine at the local welding store. Replaced all the wear items plus added a new regulator, mid-sized bottle of gas and a small spool of wire and I might be into the machine for $500. Still looks beat but works beautifully.
Found an old Miller 330 A/BP TIG machine for $600. Tested it as a stick welder; runs the smoothest arc you're likely to find in any machine, new or used! When all is said and done I might have $1K in that machine with a new regulator, foot pedal and water cooling system.
For MIG specific features I'd be looking for:
1. 220V single phase input power
2. ability to feed .022 solid wire for bodywork
3. ability to feed .030 for general fab work - bumpers, sliders, cages
4. gas capable
5. readily available consumables - contact tips and nozzles are available from Lowes, HD, NAPA and the local farm store for Miller and Lincoln machines (and their clones). Oddball machines require a trip to the welding store and maybe even a special order and a return trip.
I can also add that welding can be addictive. You get the first machine, learn it then figure out that you want/"need" another/different machine. Or you want to learn another process IE you learned stick now you want to learn MIG or TIG. So you go buy more machines and accessories...and pretty soon you need a new workshop for all the cool tools.

Not that that's bad or anything....
Nick Jennings