Julie,
Lots to think about. 6x7 cameras tend to be big and heavy. I have a Pentax 67 and Nikon F100. If you can handle the F100, which is fairly heavy, you can probably handle the Pentax 67 as well. The 67 is bigger than the F100 and a little heavier, some people might call it a beast when compared to the weight of a Nikon N80 which I also have, but I never thought of it as a beast. The Pentax 67 could become a little tiresome if you were handholding it for lots of shots during a long day, the lenses are also bigger and heavier than 35mm lenses. You may need a more substantial tripod for the Pentax 67, I don't know what you are using now, that could also add to the weight. I have the 45mm which is a great lens, but have never used the 105 or 90. The 45 may be wider than you imagine. There are forumlas for converting these to 35mm equivalents but the shape of a 35mm negative is different, a 67 negative is much more square, and to me, the 45mm Pentax 67 lens looks a lot like a 20mm lens in 35mm. There is also a 55mm lens that is probably as good as the 45mm. I also have a 75mm, which is about like a 35mm lens in 35mm and I use this lens more than any other for landscapes. I also have the 165 2.8 which is about like an 85mm in 35mm that I use a lot for portraits and sometimes for landscapes. The 90 and 105 are more or less "normal" lenses for the 67 and I can usually get away with using the 75 or 165. I replaced the screen with a Maxwell screen and liked it a lot better than the standard focusing screen. Places like KEH often have Pentax 67 bodies with brightscreens, they will also let you return the camera.
Good luck
Doug Webb