I would agree that if you want to stick with 35mm there is no reason not to stick with Monolta so that you can keep your existing lenses.
For medium format I have always taken the view that if you are going to do it you might as well go the whole way, so I use 6 x 7 rather than 6 x 4.5 (I don't really like the square format 6 x 6). For landscapes a range finder might be a good idea, but if you want to use an SLR, something rugged with a mirror lock up would be the best option. I have an RB67, but they don't like water and I tend to regard it as a studio camera. A Pentax 67 would work well. You might even consider skipping straight to large format.