In shopping around I'm learning a lot. Mostly focusing on used Lincoln and Miller tig machines which are made in the US. Everlast is a US company whose machines are made in China and they seem to be one of the better Chinese machines.
If you're welding lighter gauge steel you can use a lower amperage 110V unit as steel doesn't require as much energy to weld. Aluminum, on the other hand, in addition to needing AC current when being welded, needs a lot more amperage to weld it so a 110V machine would be limited to thin aluminum sheet.
I'm planning on scratch building a fuselage frame, something I've already done with Oxy-acetylene but which I want to do with Tig this time. There are also a lot of fittings to weld and they come out so much better with Tig.
Anything you buy to be able to weld aluminum will also do steel but not the other way around. There are DC only machines that only weld steel.
Newer "inverter" machines use less electricity and are able to weld aluminum with a tighter, smaller bead due to their variable and higher frequency AC settings instead of just the fixed 60 hz household standard. The older transformer based machines were limited to a wider bead when welding aluminum so shops that do more aluminum like the inverter machines.
If I had to pick a machine tonight it would probably be this one
Square Wave TIG 200 welder for AC/DC stick and TIG welding uses Square Wave Technology.
www.lincolnelectric.com
but there are good used machines like the Lincoln Precision or Square Wave or the Miller Syncrowave that cost less, deliver great value and reliability, but without the variable AC frequency feature. They'll still AC weld aluminum just not with the finer bead and control of the new machines. The older transformer machines are larger but super durable, the newer inverter machines can die without warning and can be expensive to fix.
That's all I've got!