Was also a fan of the personal computers in the late early 80s - the Apple II and the ZX81 were the ones that got me into programming and indirectly led to my career in engineering.
Years ago I wrote a (very simple) OS for the PC platform from the ground up. Learning about protected mode memory management, drive controllers, filesystem handling, network cards etc. was extremely educational. Certainly encourage you to find a platform that has hardware documentation and getting even something basic working, particularly if you can repurpose the hardware for other uses if / when you move on.