If you're building from scratch and have the coin, get a good Miller or Lincoln tig welder. If you're modifiying stock ones, a mig will work if it's a decent unit, maybe a Miller 140 for the low end, a 211 would be better though.
Flux core will splatter a lot and make it look crappy, so fixing that will require more time to grind off the turds to make it look better.
I'd definitely check CL for one locally, I sold my Miller STL 150 earlier this year, guy got a decent deal on it. Kinda wish I'd have kept it, but it had been sitting since I got the Syncrowave.