If you want to get some affordable z80 hardware, pick up an TI calculator. I recommend the TI-84+ Silver Edition, the other the better if you want more RAM. 15 MHz CPU, up to 128 KiB of RAM, 2 MB of Flash, a USB OTG port, a 2.5mm stereo jack (can be used for audio or networking), crystal timers and an RTC... You may have to do some work to port CP/M, though. I recently put together a simple bare-bones operating system template for TI calculators for reference:
I feel the need to also mention your OS that isn't CP/M, but does run on TI calculators: KnightOS[0].
It's fantastic, and fairly featured, and caused no end of issues at the school where I was sysadmin'ing. The kids wrote some damn good programs and games on it. (Mostly with kpy).
https://git.sr.ht/~sircmpwn/hiti/
WikiTI is a good reference for the ASIC's behavior:
http://wikiti.brandonw.net/index.php?title=Calculator_Docume...
The pages under "TI-83 Plus family" are what you're looking for.