Grant Searle has been prolific when it comes to modern builds of retro computers. On his web site you will find examples of Z80, 6502 and 6809 based machines. All with schematics and details on how he built them.
His work has inspired other to have a go at building a simple machine with the minimum of parts (The Z80 uses 7 chips).
The only problem you really face is which to build. He has a solution. Build the Multicomp. This machine is based on a cheap FPGA board that uses free tools from Altera to build a machine that emulates any of the above processors and wraps a machine around them.
The word “emulates” is a bit of a grey term here as the machine is implemented in hardware. This is not the same as running a Spectrum emulator on your WinPC. The logic needed to “be” a Z80 is programmed into the FPGA at build time and there it sits, being a Z80. There is no other software “tricking” a Spectrum program into running. The only software there is running on the bare metal.
Mine has been built as a Z80 with 2k of RAM (this saves an external chip but upto 64kb can be accessed). I have two serial ports and a VGA port. These can’t all be accessed at the same time but it’s very easy to reprogram it to use whichever of the port you need.
I’ve used a small piece of blob-board to sit on the FPGA boards I/O pins to give me something to work on. The serial outputs of the FPGA are not at RS-232 levels and so I bough two, cheap, level converters built into 9 way D-type sockets.
Get out there now and build your own. Please.