Rob, have you considered a TLR MF camera? Advantages are that they're usually a lot cheaper than their SLR cousins, lighter, and don't suffer from mirror slap, because they, well... don't have a mirror. They use a 2nd lens for viewing through ground glass (similar to an SLR like Bronica or Hasselblad). Disadvantages are that most of the TLR's have fixed standard focal-length lenses and suffer from parallax error when doing close-up work (ie. 10-20 inches away from the subject), which needs to be compensated for.
There are a couple of options available for TLRs which do have interchangeable lenses such as the Mamiya C330 or C220 - it's about the heaviest TLR you can get, but with an excellent range of lenses to choose from (55mm up to 250mm), it's the most versatile, and still about half the weight and overall size of say a Mamiya RB/RZ SLR.
Here's a pic of my C330 to get you drooling/contemplating
They're usually around £150-£200 with a lens, and lenses goes for £80-£150 each.