Do not pick one from the mainboard.

Just write the program, and use existing (hardware) board (mainboard). On mainboard you have all the external chip's, all the hardware you need.

Maybe you must design only expansion boards on ISA bus (ISA cards with some IO interface). Or just use existing IO interfaces, parallel port, serial port's, and s.o.

So, you have "basis" to experiment with programing on 386SX, just for the begining. After, you will create your own "mainboard".