If I were really looking to get in to large format, (which, if that's where you're thinking of going eventually, I'd just go straight there and skip the medium format) I'd get a nice inexpensive 4x5 field camera that can take a bag bellows, like a Shen Hao HZX-AT II (why the alphabet soup I have no idea, but it's a great little camera) and a 90 mm like a Rodenstock Grandagon 90mm f6.8. The f6.8 is a good compromise between the compactness of the f8 lenses and the cost and weight of the f5.6 lenses.

If you're still looking at medium format because you really want it, go take a look at the Mamiya website to see the range of focal lengths available for the RB/RZ line and the 7. As mentioned previously, the RB/RZ goes down to 50mm, and then has a fisheye 37mm. The 7 has a rectilinear 43, 50, and 65 as well as the normal 80. The KEH website is as good a place as any to see a list of what's available for the Pentax, although at any given moment in time they may not have any of the extreme lenses at the superwide and/or long telephoto ends of the spectrum as they were always specialty lenses and are rare.