Mig welder suggestion

Check out Eastwood. I bought their 135A mig setup with helmet and cart last year for just over $450, and it's worked great. I believe they're made for Eastwood by a company in Italy. It says it's only good up to 1/4", but with multiple passes it'd handle 5/16 just fine.

They also have a 175A setup with the helmet and cart for $650, but it sounds like that's out of your price range.