[PvK]

I think the "ghoul rate" is constant for the same priest level (until you run out of population).

All three types have their advantages, so it's up to you. My feeling is Sombre's guidelines are generally good for which to reanimate where in generic circumstances, unless you know you want a specific type.

Yes, corpses limits the number of soulless available (and whether the corpses were created by dying troops or population determines what kind of soulless you get), but has no effect on the rate of other types (I believe).

Yes, items that raise priest level (like Sword of Justice) do increase the reanimation rate, as well as possibly giving you more types of units to reanimate (e.g. Longdead Horsemen are very, er, nice but not available to an unboosted Mound Fiend...).

PvK