I have Pentax 6x7 and Mamiya RZ 6x7 systems. Of the two, the Pentax is definitely the better one for landscapes. For RZ, you have a 50 mm ULD and 37 mm fisheye to choose from. In Pentax, there are 37 fisheye, 45/4 and 55/4 lenses. The 45/4 is as close to ideal as possible. I would recommend that you get the P 67 II body, even if it is a bit expensive, as it has all the features that will make your life easier for landscape shooting. The RZ will of course work, but it is twice the size and the working method is quite different. You can put the AE finder on it, but that makes it only bigger and heavier still. As I have said in another thread, I fit a Pentax 67 II body with 45/4, 55/4, 75/4.5, 90/2.8 and 135/4 lenses, plus handgrip, set of extension tubes and a few boxes of film into a Lowepro MiniTrekker. It is relatively portable that way, if you are an average+ guy. I am not particularly strong and I can handle it. You will need a good tripod for either. In the case of the Pentax to tame the mirror slap and shutter bounce, and in the case of the Mamiya to put your mind at ease that the beast is not going to topple over into an abyss or mud pond etc. If you shoot verticals, the RZ67 and RB67 are great, as the back swivels through 90 degrees. For the Pentax, you'll need an L plate or a really good tripod head.

I use 645 Mamiyas too, and they would be great for portability for landscapes but I haven't got a really wide lens for them yet. A 35 would be good, and a 40 would have been perfect except that it doesn't exist. My widest is a 45/2.8 and it is okay stopped down but vignettes very badly wide open. The lens availability hampers their suitability for landscapes, but for most other general purpose photography the 645 is a sweet format indeed. One can easily print 24" and larger from it. If you want to, you could invest in a Pentx 645 and 67 combination. The 67 lenses are compatible on 645 via an adapter that allows full aperture metering etc.

Did you ever use Fuji Acros? Before using CMS 20, try comparing Acros with it. The Acros will develop perfectly in most if not all developers, including Rodinal, and is a ton cheaper in 120 format too. The grain from a 645 negative doesn't show up by 20" yet. How large do you want to print?