Carsten,
If your budget allows, the Distagon 35mm is the better choice for your intended use. It is more expensive than the 45mm, but it will hold its value longer due to its wider and more desirable focal length. Both lenses are highly flare resistant due to the Zeiss T* coatings.
The signature of both of these lenses, in terms of color rendition and tonality are almost identical. The major difference is really in the DOF. The Distagon 45mm is capable of a shallower DOF than the 35mm. Also, the 45mm does not hold the edge and corner detail nearly as well as the 35mm.
Since you are coming from a 35mm background, maybe a comparison with the Contax Zeiss Distagon 21mm lens for the RTS cameras might help. The Contax Zeiss 645 35mm lens is really the 645 version of this famous lens. It is the same design with roughly the same angle of view, and made with the same materials and coatings. And, even though the MTF chart looks very different, the larger 645 format compensates perfectly. In fact, I believe Son Minh Pham may have done side by side shots with both of these lenses showing the 645 version to be every bit as sharp and detailed, but of course capable of larger prints due to the format.
Another option is to consider the 45-90 zoom. In my opinion, the 45mm end of this lens is actually sharper than the prime, if you do not require the extra speed of that prime. I think this lens costs as much as the 35mm, but you may get more for your money. You will get perhaps the sharpest normal focal length in any lens lineup. At 70mm, you get a normal lens that is extremely sharp and with no distortion. And at the 90mm end, you get a nice 1:4 macro as well for convenience. This may be the only lens you need for awhile. I know fellow photographers who shoot entire weddings with just this one lens.
As far as the back, long exposures is going to be a challenge with the Kodak back. I used to own one. That back does not have the thermal engineering to support long exposures. You should be able to find a good deal on a more modern used 22MP back. If not, you may be better off shooting this camera with film for awhile, until you can afford a better back. I do not recommend the Kodak back for what you want to do. That back is notorious for developing bad pixels over time.