You don't mention the subject matter that you'll be shooting and so much of equipment choices depend upon that and your style of shooting. Are you intending to shoot B&W portraiture, E6 landscapes, or C41 street grabshots? Or trying to cover it all?
Given that there are amazing film emulsions and lenses that can exploit them nowadays, there is much to recommend considering 645 format as the logical step up from 35mm for a very substantial improvement in image quality. Unless you routinely need prints larger than 20x24 the differences with 6x7 SLRs may not be worth the substantial penalty in weight and bulk (especially once you start tricking out an RB with accessory finders, etc).
My Pentax 645N, for instance, has an integrated prism that is very bright and beautiful, integrated matrix, CW, and spot metering, instant return mirror, 2 fps autowinder, and the widest array of lenses available in MF (being that it also takes the P67 lenses with full-aperture metering, with an adapter). It is eminently handholdable down to silly-slow speeds with virtually no mirror slap. With the exceptionally sharp 35mm SMC-A f/3.5, it weighs almost exactly what my Nikon F5 and 17-35mm weighs, but with a neg that's nearly 3X as large in area with less cropping for standard sizes. These bodies are now commonly selling for sub-$300 on eBay, providing amazing bang-for-the-buck.
There's a couple of sweet spots for 645 and I've found one with the 35mm lens (that is the equivalent of a 21.5mm in 35mm terms). The near-far DOF works in a way that's very hard to replicate in larger formats with 3-dimensional subjects. The lens resolves down to where it is mostly film limited in moderate contrast light with color slide film like Astia (something that is difficult to achieve with larger MF lenses). So at least for what I'm using it for, this combo is a near-perfect fit for a niche need in my array of cameras and optics. That all said it would really pale compared to my 35mm gear for fast action or event work.