The SAMD51 has plenty of horsepower and memory to run emulators of various retro hardware, probably faster than the original systems. The Metro Grand Central M4 Express has an onboard microSD slot that's the final piece needed for RunCPM, avoiding the need for an SD shield.

The focus here is on getting RunCPM running on the Grand Central, so there's been a very brief introduction to the Z80. For more, check out the wealth of material at z80.info. The author wrote a more detailed 2-part introduction to Z80 hardware and assembly language in issues 7 & 8 of Hackspace Magazine. A couple books worth reading, if you want a deeper dive into the Z80, are Build your own Z80 computer. Design guidelines and application notes by Steve Ciarcia and Programming the Z80 by Rodnay Zaks. Both are now freely available at the provided links.

While it may not be directly applicable to most of what you find yourself doing, it can be instructive to delve into the low level workings of processors. It will give you a deeper understanding of how computers actually work. Using older 8-but CPUs for this simplifies the exercise compared to delving into the working of a modern CPU or MCU.