8051 variants with three or more analog input channels are easy to find. I'm not so sure about parts with more than two UARTs, though. If there are none, then the approach suggested by the app note should work fine.

You might also consider using a quad UART (four UARTs in one package) with the 8051, and "waste" the microcontroller's UART and the extra UART in the quad package.

With that approach, the code for all three of your devices could be similar. If you use two different kinds of UARTs (one in the microcontroller and a different one in the external package), then you would most likely have to write, test, and debug two sets of low level driver functions to talk to the two different kinds of UARTs.

This sounds like yet another cost-of-firmware vs. cost-of-hardware tradeoff. If it's a low volume project, you'd try to make it easy on the firmware guy. In high volumes, you'd go for the minimum hardware cost.

-- Russ