My M4 (non pro) Mini buying guide:
Storage 256 or 512. 256GB is a pain but doable, 512GB is easy to live with but $$$$. For both get an external 1-2TB SSD for things like Music/Photos.
RAM stay at 16GB, if you need more, upgrade to the Pro.
10GbE upgrade is reasonably priced if you are likely to use that or 2.5GbE. Certainly cleaner than a T3 dongle but not needed.
Who should buy? M1/M2 Mini owners. They are still reasonable machines, but the M4 is a pretty nice bump.