You only need AC for aluminium welding, I've got a DC one (a cheap Chinese type combined plasma cutter which works very well), wish I'd got an AC/DC one now though, but if you're not going to be welding aluminium not a problem.

If I was buying one now I'd want: current up-slope/down-slope setting or one that has current variable via a foot pedal as getting to the corner of a metal for example you really need to be able to reduce the current to prevent blowing holes from too much heat.