It is fun: but I like this kind Bonzai tree thing which is not pactical but a relaxing but challenging experience. And it feel really nice to actually really understand everything for a change.
I grew up in the 80s with 8 bit computers and started collecting them in the 90s-2000s (when you could pick them up for free as long as you picked them up); I started repairing and extending them picking up video cards and old mobo's from dumps and 2nd hand markets. You can still buy boxes with mobo's from < 2000 which have usable things like memory for much older systems for next to nothing. Using a cheap digi scope, some bread boards, wire and analog electronics can get you quite far in playing around with slow (70-80s) digital components and hooking them up to others. On ebay they sell kits with old processors as I wouldn't recommend soldering apart 80s computers anymore => they are collectible and much appreciated in others or my museum.
That said, it is a good start... If you can find a cheap one, find the schematic online and start checking out the components and measuring the signals. I am lucky to have many systems that have 74series logic + memory + cpu in them which is really easy to follow for any programmer. Most machines from that era won't have that unfortunately.
If you cannot or will not risk that then simply buy a power supply, some packs of analog electronics, digi scope (few $100 one is enough), a box (2nd hand you can get them cheap) of different 74series, breadboard and a computer kit like [0] or [1] or [2] and start experimenting.
This kit seems very similar to the famous Altair computer which spawned MS-BASIC and CP/M. Anything like Unix and C is not desirable on such a system. Even a 'text editor', as we understand them today, is a reach on such machines.
You could build your own world, but you'd probably have to do it writing ASM out by hand and then entering through a cumbersome line editor. For most purposes, that's too hard, so you would probably invent a very 'basic' line-level interpreter. But, given what we know now, you'd probably come up with a better disk operating system.
It's actually pretty surprising how much you can achieve with these 8 bit processors. My favourite example of pushing the 8 bit Arduino processor to the limit is this - 3D rendering at 320x240 x 16 bit: http://hackaday.com/2016/01/02/better-3d-graphics-on-the-ard...
That demo is rather impressive, but the AVR in an Arduino is much faster than a Z80. This machine is running at ~7.37MHz (which is fast compared to old Z80 based personal computers) whereas the Arduino typically runs at 16Mhz. Further, an instruction takes a minimum of 4 cycles on the Z80 and many take longer whereas most AVR instructions take only a single cycle.
Just a C compiler and a text editor and needing to build a world around me.
Would be fun.