You might want something like i5-13400F (or KF) or Ryzen 5 7600X, those seem pretty good for the price. The F suffix means a need for dedicated GPU (does not have to be powerful) and K allows overclocking.
Note that the desktop 13th and 14th gen i5 CPUs should not be affected by the infamous Intel high-end CPU problems.
I have no experience in using high performance desktops on Linux but I think those CPUs should work well (AMD may be better).
I’ve been running Nvidia on Linux for over 15 years, with the proprietary drivers that come with Ubuntu. It has been a pretty smooth journey, with only a handful of hiccups.
Anyway, with RawTherapee, the GPU won’t be used. Unless some other software needs it, it’s not so important.