Start with a known platform, I suggest a RasperryPi (because it's cheap, plentiful and capable of what you want to do). Then assemble the emulators required and the write an app that ties the emulators together, in the way you want. It will essentially be an app launcher, which perhaps can offer a "BASIC" command line OS function.
-edit- perhaps you could start your project with this:
http://cbmbasic.sourceforge.netNext build the cart interface, I would suggest using a microcontroller like the Teensy 3.1, as it has enough IO to interface with an 8bit Cart and can send the data over USB, which will be much easier to interface with your software part should you change platforms.
-edit- I forgot to mention that the Teensy 3.1 is also 5V safe, making it perfect for interfacing with old Electronics from the 80s.
Sounds like a fun hobby project. Certainly, it has no commercial value.
