This is exactly the same as you're asking in your other thread, the devices are UART's (as before), and the easiest (and cheapest) way is probably to use PIC's to make them.
Yes, but why would you want to do that? (As depicted in your diagram) It would be better to use the PICs built in USART.
Most PICs have an ADC built in, but you'll need a DAC or use the PICs PWM and a Low Pass filter to go from digital to analog.