what is the least expensive welder i can get away with ?

These recommendations have been covered but I'll second or third or whatever them...

Get one set up for 75/25 shielding gas and make sure it has both continuously variable wire and heat settings and not the just the click stops.

If you can't afford a Lincoln or Miller rent or borrow one for the job at hand. Spending money on an inferior welder is like flushing it down the toilet.