A MIG for the track is probably the most versatile. Miller introduced a new "hobbyist" hard wire box for under $600 and will run 110, maybe 220.
Their MultiMatic 215 is a nice unit. D.C. TIG (lift arc only) but foot pedal, MIG and Stick. Also can plug in a spool gun and MIG aluminum. Runs 110/220. Very small and lightweight but welds really, really weld. I used mine (I have the prior 200 unit) today welding up a pontoon boat with it for a buddy. Carried my Dynasty to TIG some parts, but MIG'ed up 3/16" aluminum easily.
Downside to TIG at the track, the area to be welded has to be super clean. MIG will tolerate some dirt, and when you're trackside, who has time to clean when you're making repairs? But if you want a portable Stick thrown in, these would be my top recommendations.