Where do I start? I looked at adding a CNC kit to a mill myslef, but most
affordable mills are to small to do some of the work I want to do. Am I
back to looking at the rotary tool on rails setups?

Mills with obsolete CNC controls can be found for scrap price. Look for a
knee mill with a BOSS (Bridgeport) or Bandit control. If you're near the
left coast, there seems to be a lot of Shizouka mills, one of the best
choices.

Refit the machine with the PC based control Mach 3, This is a bit of a job
but there are several NGs to go for advice. CNCzone.com is one of the best.

Good luck

Karl