Actually the 5/15 and the 5/25 both have a 15 pin d shell port, but hey are DH+ ports. The only way to get to an RS-232 port on the "classic" processors is to add another card to the system - both rockwell and pro-soft both make serial port
modules for the PLC's and SLC's. The 5/20 5/30 5/40... line of processors do have serial ports (25 pin) as well as DH+ and RIO ports.
No, it does not. You have to go up to an "Enhanced" PLC-5 to get the RS-232 serial port. Your best bet - Get a PLC-5/11 (it is probably the cheapest version enhanced processor AB sells).
