Unless you plan on spending hours on the trapeze, you don't need the full auto harness or even the convertible harness.
To be honest, even the skipper's harness feels bulky and uncomfortable on the trampoline. If you're going to wear it, you're best off on the wire. I think I prefer the "old style" or "economy" harnesses, made from a single piece of fabric. They're cheap ($40-50) and easy to wear, and work fine if you're not spending great lengths of time on the wire or are in decent shape.
However, I don't race, or sail from the trapeze much. It's a bit complicated to try and get out on the wire while holding the mainsheet, traveller sheet and tiller, so I leave the trapeze work to my crew.
Do you know what brand those $40-50 harnesses are by any chance? Or maybe some retailers that sell them? I was watching one cheap harness on eBay that was going for $30 but I let it slip by.