Hi, I have worked with the 5D (1) and own the 5dm2, I've owned the 1Dm2 and worked heavily with the 1Ds m2, and awhile ago I bought the Sony a850 as a backup (I didn't need it, but I wanted to see if it was all I was told, and it was so cheap), which is making think of you at the moment. I know this is a Canon forum, and most people think "Canon or Nikon", even from clients I've seen it, but for your uses I suggest an interesting alternative, the Sony a850. Yes it is full frame.
Before it goes ignored, the viewfinder is stunning and feels so much larger and crisper than my 5dm2 (mild yellow tint, only notice it a rare occasion), it's the exact same 24mp sensor as the a900, same rugged as hell body but about the same size as the 5D. Remember Sony bought Minolta, the camera feels very Minolta (chunky, lots of neat touches, rugged, probably doubles well as a hammer)
The a850 was sorta a forgotten camera, it came out a year after the a900 and 5dm2, in their shadow, but it cost less and only suffered a slower max frame rate (you shoot view, I doubt 3 frames per second is too slow for you), and a viewfinder at 98-point-something percent instead of the full 100% (I can't see the difference, it's still stunning). Because it doesn't have the a900 title and began selling for cheaper, I bought one as a backup for $850. That's $850 for a 24mp camera, with (to my eye) equal noise control and DR (many may argue, but I think my 5dm2 holds better shadow, while my a850 picks up better highlights, the differences are small, you may not notice, and yes I only shoot raw).
Lenses, because of the Minolta mount you can of course use new Sonys, but also the gamut of used Minoltas, which are very cheap and since you seem to like manual focus I doubt the slightly slower AF rate will bother you. You want primes, so I'm thinking they'll be cheaper than Canon, Nikon I admit some of the old MF AIS lenses have proven amazing (nikkor ais 55mm 2.8 micro may be my favourite lens ever) that you can get cheap too, but being that you want to print big, 24mp does the job well. I'd use it as my main camera if I'd invested more in glass, but since I have a 1dm3 for field work I invested in fast AF canon glass, the sony a happy backup I often use when I go hiking.
Final note, the screen, I'd say it's vastly VASTLY superior to both the 5d and 1ds m2, I know you said that isn't so critical but I remember having many issues with the 1dm2 series in bright sun (like any screen of the era, not their fault at all), coming back to realize how much cranking you need to do in Raw can be depressing, even the best cameras suffer fine quality loss/distortion when pushed hard.
Anyways, you were worried about an old camera, the a850 is four years newer, it's seriously an a900 as far as you're concerned, built to compete in the era of the 5dm2. If you do have to stick Canon, you already carry a view camera so the size of the 1Ds m2 shouldn't bother you, it's excellent, though remember the weatherproofing doesn't exist without weatherproofed lenses (water straight into the mirror box with the sensor is basically the worst place it can get), so waterproofing requires a more sizeable investment. As for your comment before about the x100, own it and I do adore it, but not being able to change lenses kills it for anything other than personal fun, also the manual focus is horrendous... honestly.
Good luck!
Dimitri
