Unfortunately the three things that you listed (trailer frame, body panels, exhaust) are some of the most difficult types to master. The exhaust and body panels, particularly the body panels, are very thin and thus, very easy to burn through. The frame welding is getting into the realm of life threatening consequences of not doing it right.

Of all the technologies, wire feed is the one which is the easiest to have look really good and not be worth a damn. It actually is pretty hard not to lay a nice looking bead with a wire feed machine.

As hinted above, I would get a bunch of scrap steel and practice, practice, practice. And then cut your welds apart and see how well you did. u My local community college is much more enlightened, so I took a total of 5 quarters of welding (stick, out of position stick, wire feed, oxy-fuel, and TIG). I was also once of twice per week using the stick welding at work. Each time I welded I became a bit better, but only if I were welding regularly. I haven't done much for a few months, so I would have to run a few practice beads before I would attempt anything important or pretty.