I used to have a ZX-81 and started to tinker with the I/O bus, long story short I shorted something and fried the CPU, so I went to order another one (since it was socketed it would be just a drop in replacement) well, back then you will have to wait months to get a part, their estimate was 2 months and it took 3, but they did have a book in English (this was in Spain early 80s) about the Z80 assembly language.
I did buy the book and while waiting for the actual processor I did all the exercises by hand (I was pretty much the CPU).
The target device for the book was the microprofessor MPF-1, of course there was no way I could afford it back then but I did learn assembly for my 1st time using that book ( I think it had yellow covers but not sure anymore)
I always lusted with the thought of purchasing one, but instead I used the ZX-81 and later on the ZX-Spectrum to do my assembly code, (I was the assembler btw, figured out the op-codes and poke the code using Basic) lots of fun for me at the time
Anyways, I don't lust upon it anymore, you can get a full soft core z80 fpga but what for? Current MCUs are way past what those things could do.
I do have somewhere an assembler and debugger that did run on IBM and clones, but as soon as I saw it boot and the copyright for the Spectrum I lost interest because I got it working. The thing is that my energy (other than figuring that out) moved towards the current generation so I moved on.
Not saying you shouldn't seek an MPF-1, but why?
I just noticed I already told my story earlier on this thread, well, less detailed but still. Move on to current gen. Even microcontrollers have more computing power that that awesome z-80 (well awesome at it's time with their expanded instruction set over the 8080).