I'm not a professional welder, but I'm pretty good and keep my eye on the market closely. I've been considering adding a plasma cutter to the shop for quite some time (I have a Hypertherm at work I can borrow as much as I want). I'm going to buy a Primeweld CUT60, which is a touch more than your budget, but they also make a CUT50 which is $400, 50A, dual-voltage unit that has an AG60 torch (pretty common).
Primeweld was started by a couple of guys from the big name welding companies and they are located here in the U.S. They have the machines made to their specs overseas and their customer service is pretty legendary. You can get someone on the phone in a couple of minutes and I've seen folks claim they got a reply to an e-mail over a weekend. They have really taken the affordable TIG welder market by storm the last few years. I have three coworker/friends who have bought welders from them, and the guy who sits across from me just added the CUT60 to the TIG225X he already had....loves it more than his old Thermal Dynamics unit.
I would definitely recommend looking at Primweld over the YesWelder stuff...in the unlikely event you have a problem, it'll be a lot easier to deal with. The most recent customer service issue I saw with a Primeweld that got damaged in shipping and they shipped a replacement without even waiting for the damaged unit to be shipped back. It's that kind of company.
The PrimeWeld CUT50DP 50-Amp Plasma Cutter is portable and uses compressed air and does not require a shielding gas. Order online to get free shipping.
primeweld.com