Zzzop, a 12v starter is plenty adequate, while a 24v will make it seem even easier. One of our old logging trucks contained a 903ci Cummins, which is considerably larger than the D4's engine and the 12v starter in that engine had no issues starting the engine in the dead of winter. It helps obviously that the 903 is direct injection and there are more cylinders to help get going but applying external heat like you're planning is the key to easy cold starts even on old clunkers like the D315. The block heater on my F350 makes starting it in winter just like it it was already running and warm. Just make sure you don't skimp on the battery cables. Use 00 cables, new clamps nicely soldered, clean and bright connections, and you won't have any issues.
I contacted a starter motor guy about building a 6v MT style starter for my 5U. He was quite interested in it and I really must get around to following up to see what size cabling I'd need. Existing cabling is 00 to the pony starter but the current draw on a 6V MT would be easily double that I'd say. I have a single 800CCA battery currently : ), and could easily fit two. My idea was to use the 6v starter for when I "accidentally" stalled it on the heap at the show in front of a bunch of girls, so as to save some face! I think it would have just enough to start the main when hot, and after getting it spinning over on no compression. All of this is just so's I don't have to replace the 6v generator. It's not the best technical solution obviously but it's one of those "I wonder if it would work" scenarios.