Ummm... I should probably point out that one
cannot adapt Pentax-K mount to Nikon-F the way you can to Canon EOS.
Nikon's registration distance is larger than all the other SLR mounts except for Leica-R. Registration distance is the distance from the mount to the sensor. This is the distance the lens has to be held to achieve focus to infinity. You increase this distance, and it's like adding an extension tube, and you can't focus over the entire range. And decreasing would mean jamming the lens further back into the camera, which is very difficult. You can get an adapter with a glass element in it to act like a short teleconverter, but the cheaper the glass, the lower the image quality.
Adapting to Canon EOS is super-easy because it has one of the shallower registration distances. Adding to that distance with a simple adapter ring to make up the difference with other mounts is easy. But Nikon's got one of the deepest. If you want to adapt a manual focus lens from another mount to Nikon F, you're going to have to shave off distance from the lens mount. And the only easy solution for this are the
Leitax kits, which are offered only for Contax/Yashica (Zeiss!), Leica-R, and Olympus OM mounts at this time, and they will not cover all lenses in the given mount, due to differences in how far the back element sticks back into the camera body.
Also, on the Canon side of the fence, the clearance between the back element and the mirror is smaller on the 5Ds than on the crop-body (and 1 series) cameras. It might be similar on the Nikon side of the fence (it might not). I weep that the C/Y Distagon 28/2.8 I loved so much on my XT and 50D whacks into my 5Dii's mirror if I focus it to infinity; my C/Y Tessar 45/2.8 is a complete no-go on the 5Dii, and I had to grind down the shark fin and aperture lever on my C/Y Planar 100/2 to get it to mount at all on the 5Dii. OTOH, my Olympus OMs and Leica Rs have given me no trouble at all.
If manual focus holds no fears for you, Mars, some lenses you might want to consider are the Samyang/Rokinon 84mm f/1.4, 35mm f/1.4, and 14mm f/2.8. They've got some insanely good reputations for the super-low pricetags they sport. And, of course, there are like a gazillion fantastic old Nikon F manual focus lenses you can use without mucking about with adapters. Just avoid pre-AI lenses, or learn how to grind down the appropriate flange to avoid damaging your camera. Myself, I'd highly recommend playing about with a 50mm f/1.2.