Daywalker,
I've only done one, and that was a simple Airmodel Bell X-1 I picked up on ebay for a couple of bucks. Didn't even bother with the landing gear, just make it "in flight" to simplify it as my first vac. Despite my inbred fear of vacuform kits, it wasn't as hard as I expected. Once the pieces were cut out and sanded, it really went together like a short-run injected kit. It gave me the confidence to order several more vacs of hard to find subject that interest me, most of which are only available in vacuform.
Others involved in this GB can certainly give more expert opinion than mine, but I'd say for your first you may want to consider a simple Airmodel or Rareplanes kit. They are very basic, but seem to be of good quality, and can be had for a couple of bucks on ebay all day long, so if you screw it up, you won't be out much. I ordered several more from one seller on ebay to save on shipping, picking up a Fairey Fulmer and Curtis CW-21 Demon from Rareplanes, and an Airmodel Fokker G.1 for a couple bucks each. I may try one of these for the GB.