Chris don't buy one any smaller than 220V- 180 amp or you won't be able to weld very thick Alum. Lots of good units pop up on Craigs list that are almost like new because people buy them and realize how hard it is to tig weld Alum, give up and sell them.
I paid $1200 for an almost brand new one when I was shopping.
The other option is to buy a good sized Mig, that is capable of adding an Aluminum spool gun to it, then you have a dual purpose machine.
