This has probably been beaten to death but I searched and couldn't find anything on it.

What is the interchangeability of wheels from different models of Jaguars? Do they have the same lug sizes? Which are the same? Which are different?

Anything Ford-based (S-Type, X-Type) or later (2004+ XJ, XF, 2007+XK) use a 5x108 lug pattern and a different thread pitch for the lug nuts so neither the wheels nor the lug nuts interchange with the X300. Anything before that (2003 and older XJ, 2006 and older XK, all XJS) uses a 5x120.7 bolt pattern, and sometimes you will need different lug nuts; for example, I have a set of XK8 17" wheels for my snow tires on my XJR and they use a completely different set of lug nuts. The usual concerns about brake clearance apply, especially for XJRs.