Renting for a day is a good idea if you don't think you'll have other use for it.
If you want to buy, I'd recommend a Lincoln 225A "buzz box". I often see them used in the classifieds for $75 to $100. If you happen across an AC/DC version, grab it. They're pretty good welders. Nothing sophisticated, nothing fancy, but they'll glue steel together fine.
If you get a used one for that kind of price, you can perfect your skills, and when your budget permits, sell it, get your money back and buy a TIG or MIG machine.