What do you want to do with it? Welding aluminum requires A/C ability.
Things I look for in a tig machine:
H/F start.
A/C D/C- D/C+
Liqiud cooled torch. (gas cooled torches consume way to much gas, and are generally oversized and hard to manage)
At least 175 amps.
The best "add on" to a tig machine is a liquid cooled SPEEDWAY brand torch. Once you use one, you will never want to use anything else.
I prefer foot control amperage pedal. I don't like the torch mounted switches, but it does come in handy when you're climbing like a monkey through roll cages.