I use a Miller as well, the 200SD is what I would buy if I were buying one today.

You need a 240 volt machine, period.

1/4" plate is pretty thick for TIG welding....you'll probably be at max amps and after a minute or two won't be able to hold the torch any longer. Most guys using that many amps will have a bigger welder with a water cooled torch - more money, etc.

If you're going to weld a lot of 1/4" plate I would use a MIG welder or the stick setting on yout TIG welder.