Design plan for a calculator
There are many microcontrollers out there (a microprocessor is slightly different), and propably an ARM is the first choice if you want performance. Hook up RAM and processor is conceptually trivial, but the practice can be complicated if you are not expert.
RAM is probably embedded if you buy a microcontroller, so just load the software from the Flash; and wait for using operating systems, first try to play a little bit with the board.
If you don't have a specific requirement for multiprocessor systems (and you should have them in the same chip, I don't think is feasible with microcontrollers) then it makes no sense to get crazy with that kind of stuff. And if you have to learn, I would suggest again of starting with something simple, you can't think to build a computer without even have build a microcontroller board, at least once.


And probably you are expert in programming, but even if you have the PCB made, programming with microcontrollers requires the management of the hardware, and that's also a thing to learn before going further.