My question is that the RX and TX lines go to the WIFI shield, where would I bring in the RS-232 data (Serial TTL) logic level?
If you connect your RS232 to a shield, then obviously there is the possibility that you will get conflicts in the pin assignments of your other shields. You could look for an alternative shield that uses a different pin assignment. You could use an 'in between' shield to re-route the pins between layers. You could use a separate board (instead of a shield) which then enables you to connect it to arbitrary pins. If the logic level serial signal can't be routed to the hardware serial UART (pins 0 and 1) (and you don't have an Arduino with any other hardware UARTS free) then you have the option of using SoftwareSerial on any free pair of I/O pins. I understand that SoftwareSerial is not capable of keeping up with very high line speeds but I don't know what the limit is - just be aware that if you take this approach it may rule out higher line speeds.