Can I directly connect GPIO to RS232 RXD and TXD?
The specs on the N920 radio say that the serial interface is TTL, it is NOT RS232.
There is no purpose in giving either the microprocessor or the radio a
transmit/receive RS232 interface, unless they are to be separated by
a large distance.   The serial I/O terminals on the N920 radio can be directly
connected to GPIO pins.

Unless other hardware needs it, there is also no requirement for 12V.