My MIG works on 110V and welds up to 1/4" steel. I doubt you'd need more than that.

Burly Miller TIG's use 240V circuits... and you can probably find a 240V 80 gal compressor. Just use one outlet for both because you don't need the compressor while you're welding, and it will store enough air to last you until it gets plugged back in.

As for the floor... The concrete's pretty rough, and I wouldn't want to do that without having it sanded first. I'm thinking about knocking down the garage and building a 2.5 car jibbity. The slab is twice as wide as the garage that's built on it, and framing a new garage would be a piece of cake if I had 3 friends and a weekend. EDIT: But I have no friends. /EDIT It would also justify the cost of running better power (only have a 50A circuit) out there. The only thing stopping me is the cost of 2 new garage doors.