First, you are trying to do all things in one whack, that is an idea I believe is wrong; however
8051 compatible controller (@12MHz),
2-4kb flash ROM
use 11.592 MHz for the UARTS.
There is no way you can cram the code for the devices you list below into 4kb, Have a look at the Philips P89C51RD2. Also you will probably have trouble driving 5 UARTS at a reasonable speed with a 12MHz device.

32 DIO channels
2 A/D Channels (Differential Inputs @±15vDC and 8-bit resolution)
3 D/A Channels (Differential Inputs @±15vDC and 8-bit resolution)
search on I2C at www.semiconductors.com A/D +-15 is a matter of resistors to convert to 0-5.
D/A channels have analog OUTPUT, if you need +- 15 you will need an op-amp.

4 Nos. RS-485 ports.
No '51 has 485 ports the '51 has UART(s), you need 485 transceiver chips from e.g. maxim. Philips has some wonderful chips with FIFOs and all kinds of goodies with up to 4 UARTS which plus the one in the RD2 gives you 5.

Erik