I can't believe people are STILL recommending ancient old oil cooled stick welders, when the OP specifically said MIG
If you have 3 phase, definitely get a 3 phase mig, something around 300 amps or upwards. Fit 1mm wire to it and it will do everything from 70 amps to 250+ amps.
There are always good 3phase welders on the 2nd hand market, look for good brands in good working condition, such as kemppi, Miller, etc.
A big MIG set can do small stuff just fine, but a small one can't do big stuff at all.

However, if you need to weld off the 7kva genny, just get a little 160a lunchbox inverter stick welder, they're very portable and much nicer and easier to weld with than an old oilcooled.