I more or less taught my self how to weld (friends took a class, read on internet, etc) but it definitely took some time to get OK at it. I wanted to do it, so I did a little research, and bought myself a Lincoln 100 from Home Depot, just the basic flux core set up. I've now built bumpers, my exo cage, and help friends with their stuff. (yes all still with the little welder that could)
Renting one for the day just to get a project done, with no experience, probabaly isn't the best way to start. It takes time to get the finesse part of it down, and like others have said, starting with a crucial part probabaly isn't the best way to go.
Do you have any 4 wheel clubs near you? I would bet there are lots of guys (fellow wheelers) that would be willing to help you out and do it for free, or at least for a few beers.