Do you plan to shoot C-41, slide, or B&W? Do you plan to print, scan, project? Do you have prior experience with film? How big to you plan to print?
If you are intrigued by 6x7, do consider the Fuji 6x7 rangefinder offerings, as others have suggested. You can use them hand-held or on a tripod. The version II models can be had fairly cheaply and will give you a feel as to wether or not you really want to pursue film, and in what way. The Yashica MAT series are another affordable alternative in a TLR, fixed-lens, TLR format. Both are fun to shoot.
Don't underestimate the 645 format. My Mamiya 645 Pro is one of my favorite cameras. It can be used hand-held or on a tripod. They are cheap and the quality is very good. The autofocus models are a bid pricier. Add a metering prism and motorized grip and you have a rather large yet hand-holdable SLR.
All that being said, you can't beat the image in the viewfinder of the Mamiya 6x7. It is gorgeous, as are the resulting negatives.
My own evolution went 35mm, DSLR, 645 (Mamiya M645 Super), Yashica MAT-124G, Mamiya Pro SD, 4x5, and some 35mm RF (Bessa and Minolta) in the mix too.
The larger negative sizes can be seductive. In the end, you will find a format and system that fits the way you like to shoot as well as how you like to experience and share your resulting images. Be open and don't sweat any particular system. If something doesn't work for you, you can always sell it while losing little to no money. These old film cameras have already lost most of their value, they won't get much cheaper.
Good luck!