I'd go for a 645 SLR with exchangeable magazines (such as Mamiya M645 Pro), or a Mamiya C series twin lens reflex as a first medium format camera to be used primarily for landscapes in the under-$500 range. C220s are probably the best value in the C system. Lenses are cheap and excellent.
If you have plenty of money to invest, just go for a Hassy off the bat, and be sure to get a few extra backs. They come in handy in landscape shooting, IME.
I like the RB or RZ too. It is what I use most for landscapes. Before I got it, I mostly used 4x5 and a Mamiya Press 6x9. I switched to the RZ because I view quick TTL composing as the most important feature for landscapes (and most other shooting, for that matter).
I like the P67 for hand held stuff, because it is pretty comfortable to use. It's a good medium format camera for those who are in love with the feel of 35mm SLRs. I don't like it much on a tripod (especially in studio) or with flash. I prefer a more "component-oriented" system for shots when I can take my time a bit. Maybe this is just because I used to always use 4x5 for this sort of stuff before entering into the medium format world.
IMHO, you won't find a lens bad enough to warrant a second look in any quality medium format system. Hassy, Bronica, Mamiya, Pentax, etc. They're all better lenses than most people are photographers, so I don't feel that they are very often the limiting factor in making a good picture.