This is very, very like an A-level project I did at school - getting on for 30 years ago!
It was a great learning experience, though perhaps not in the way you'd expect. I had very few issues with the electrical design, but some major problems with the breadboard itself.
The main thing was power distribution. Simply daisy-chaining the power rails from one board to another didn't work, because the contacts were quite resistive, and multiple connections in series meant the voltage dropped too far. I remember that, with it in this state, adding decoupling capacitors across the power supplies of the chips actually prevented it from working at all.
Symptoms were truly bizarre, like: it had a 2 digit LED display, which would work fine until you put your hand near it, then it would gently fade to a dim '88'. Move away and it's OK again.
The fix was simple, just run multiple wires from the supply to each of the individual power rails, then decouple properly.
IIRC it ran at 2 MHz and would have had a 32kx8 SRAM, with code stored in a 27128 EPROM. Input and output consisted of a numeric keypad, an ADC and a DAC, and it was used to do simple audio effects (echo, reverb, reverse etc).
Really enjoyed that project, even if it was ridiculous overkill under the circumstances.
I got an 'A'
