Upload & Sell: On
1. Mamiya does make a 28mm, but it is a truly expensive lens. Are you sure you want to kick off your medium format (film) experience with such a thing? 645 film is great, but for landscapes "some people" tend to favor an even larger negative, such as 6x7 or 6x9, or even 4x5 and larger.
I cannot answer any questions about the 28mm besides price and compatibility, I've never even put paw to one.
2. Contax 645, Pentax 645, Fujiblad 645 - none of them will work on the Mamiya AF cameras. Hasselblad 6x6, yes, you can use an adapter to make it fit. The same goes for Pentax 67 lenses to Mamiya 645.
3. This is almost the first question I would ask, if I were in your shoes. Wide angles don't mean great landscapes by default. Frankly I think you are better served, if 645 format (film) is what you want to shoot, by perhaps trying some of the many Mamiya M645 (manual focus) lenses that litter the world. They are relatively inexpensive and work on the autofocus Mamiyas with some (minor) limitations: for example only the spot meter functions on the camera body, and you must perform either stop-down metering or calculate the exposure in your head, on the fly, based on full-aperture metering (and manual focussing) then turn the aperture ring based on that.
Honestly, if you are able to afford the 28mm AF lens - you might consider getting a Mamiya 7II and the 43mm lens to start with, as well. It's very hard to use special filters with maximum intended effect on a rangefinder camera, but the Mamiya rangefinders are amazing cameras in terms image quality.
My question for you: how or where do you plan on either printing, or otherwise digitizing your photos?
Edit: why version III of the camera? It has improved autofocus, but it's not like landscapes need AF like moving subjects do. The 645 AFd and AFdII are cheaper and can use the exact same lens lineup...