I've been gas, stick, and Mig welding for many years, but I have never had anything to do with Tig and am clueless...

Three questions:

Are any of the Tig Welders one sees on e-bay for around 300 bucks worth getting for light weight aluminum work such as mufflers, engine cases, etc.?

Also, do the Tig welders use the same 75/25 Argon/CO2 mix as I use in my Mig welder, or is it pure Argon for the Tig's??

What exactly DO I look for when shopping for a cheap Tig?

Thanks,
AV8TOR