Disregarded in this thread til so far is the mentioning of the Optika or Rittreck camera system: SLR with focal plane shutter, interchangeable backs, waist level finder, image size from 6x4,5 to 6x9, interchangeable lenses, ability to mount barrel lenses if you're handy enough to make your own lensboards. Can be used handheld. The one drawback is that the shortest lens length is 92 or 95 mm - so no wide angle landscape shots - OTOH, this same problem exists with most TLR's that have a fixed lens.
Other drawback: they don't come up for sale that often, even though last year there must have been approx. 6 or 8 for sale on eBay. You should get a decent one for around $200-300 or a slightly more battered one for $120-150. We're quite happy with ours, as it seems lighter than other medium format SLR's like the Pentax 67. You'll need muscle for that one to work with it handheld. Another thing: It'll sure make you stand out in the crowds with their hassy's and rollei's.
If you want more info on this camera, send a pm!
Good luck. The first step is the hardest. Allow yourself some room for trial and error as well. Chances are you'll end up with a couple of different mf cameras, each with it's own pro and cons. I know very few people who own only one type of mf cam these days, with the prices falling every year. Indeed, it is amazing to see the price drop in the mamiya 645 and the mamiya 67 series. Sometimes they're cheaper than a new Kiev.