If you are planning on doing a lot of fabrication I would certainly suggest going with a larger 220 volt machine. Sometimes the little 110 machines can provide the penetration on thinner items, but you will always be running the machine at its maximum and it will burn up quickly.

We repaired a car years ago that was built with a 110 welder. The welds all looked beautiful, except that the first time they let go of the trans brake the rear end spun backwards out of the car. The roll cage was also installed with the same welder. We cut one of the door bars by the floor and just popped the other weld right off the main hoop by pulling on it.

Tig machines will weld anything you will come across, but it is slow and labor intensive compared to mig welding. Not to mention, sometimes its hard enough to get a mig gun into some places on a race car, now think about getting the tig torch, your other hand and filler rod in there, your head in a position so you can see the puddle and then throw also trying to operate the foot pedal all at the same time.

As for shielding gas. Obviously with a tig you are pretty much stuck with argon. With the mig there are various mixes you can use. I have tried several of the different mixes over the years and I have always gone back to using just straight CO2. Last time I filled a large bottle it was like $25, lasts about 3-4 times longer than mix gas and Ive always been happier with the welds.