A counter-argument: buy the Mamiya 7 and 2 lenses.
Why: well, the lenses are stunning, and when coupled with 6x7 film, deliver big, sharp, prints. It's also a lovely camera to use, because it really is so simple.
Yes, you will have buy film, and pay to have prints made, given that you are not equipped to do it yourself. But unless you shoot hundreds or thousands of rolls a year, is that really a significant cost? B&W film and chemicals are cheap.
MF kit has taken a big hit lately, so hopefully you will be able to pick up a system at a reasonable price (note, though, that M7 gear seems to have held its price a bit better than some, so perhaps there's more to lose later).
You presumably have a camera (whether film or digital) with which you use your existing EOS lenses, so they won't simply be left lying around.
Best of all, decent flatbed scanners like the Epson 4990 are inexpensive, and can get you into the digital workflow gradually. Add a K3 Epson priter at some point, even an A4 model to start with (does Epson do one?), and become familiar with Photoshop, digital B&W printing, etc., gradually.
I currently use two M7 bodies, together with 4 lenses, for my main landscape work. Recently I have been experimenting with digital, having acquired a 350D, to go along with my 1V and 13 EOS lenses. Though impressed with it, I did not feel digital was the next stage in my photographic endeavours. So over the past 4 months, I have purchased (2nd-hand) a Wista DX 5x4, and a selection of lenses. Total cost has been well in excess of a 5D, but for my purposes, this has been money well spent. Not only am I getting great image quility, but the benefit of tilt and shift on all my lenses.
Ultimately it's about how you want to do your photography. The digital route, with a 5D, etc., might well be the best choice for you. But since you asked the question, I thought I'd risk the flames and offer a contrary view! Good luck!