well, are you going to be welding aluminum or anything requiring AC?
How thick is the material you want to weld (maximum you run through?)
Do you have single or three phase power available?
What is your budget?
for a switchable entry level Tig, look at the Miller Econotig, that was the first Tig I started with.
Above that, I like the Syncrowave 200 or 250.
I also like the features found on the HTP upper line of Tigs,