I am amazed that people are still interested in this... Unless you are a student and targeting to do design work in the future.
i am amazed that people still do new z80 home computer boards
though, i understand why they may do it.
and if (when) i will start with FPGA at some point i will try an implement an mcu on it
The 8080 based Altair 8800 ran at 2 MHz and the later Z80 systems ran at the blistering speed of 6 MHz. Today you can buy a Z80 chip that runs at 50 MHz and you can probably build one that runs at 100 MHz. Then add in an operating system like CP/M with a Compact Flash to implement all 16 logical drives and you have a pretty nice machine with a command line user interface. In fact, it smokes!
I still have all of the CP/M related software including PL/I and FORTRAN. I primarily use the Macro Assembler and WordStar as an editor. Anything but 'ed'... Were I truly interested, I might try to resurrect UCSD Pascal and see how it would run on a 50 MHz board. It would be awesome!
I bought one of the Zilog 50 MHz boards, added an identical sized daughter card with two USB ports for the console, reader and printer and a Compact Flash socket. This is a very nice system.
These retro computers are a lot of fun. To build them, you need decent hardware skills to design and build the interfaces (if you use a manufactured board), solid coding skills to write the BIOS and boot loader and, if you are creating the core, very solid FPGA skills. These are NOT Arduino projects!
Or, you can use the advanced hardware features of the modern Z80 chip to build a robot, running the 'bot under CP/M and writing the code in Microsoft FORTRAN! That's about as retro as it gets! If you do the core on an FPGA, you can add all the PWM ports you want. Wheel encoders are trivial.
Once again, I'm going to mention "Microprocessor Design Using Verilog HDL" because the fellow who wrote it worked for Zilog on advanced Z80 chips. Write your own Z80 or copy the T80 core from OpenCores and see how far you can get.