XBee S1 is using CMOS logic
instead of TTL logic, hence MAX3232 have to be used instead of MAX232.
MAX3232 can work at 3.3 but MAX232 not.
Also, the manual stated that the minimum connections for updating firmware are VCC, GND, DIN, DOUT, RTS & DTR
You may want to have a look to the interface board schmatic:
Regarding to the main problem, XBee S1(Before REV.
is composed by Freescale MC13193 transcever and MC9S08GT60 MCU
, hence XBee itself is strong enough to work without additional MCU. However, you have to work on the code yourself on Freescale BeeStack or TI Z-Stack.
But, if you just want to make a simple point to point network, using the UART interface and AT Command would be good enough. You may add a cheap MCU that work on CMOS logic to control the XBee, no MAX3232 will be needed in this case.
If you still want to write your own code for the MC9S08GT60 inside the XBee to make a simple point to point network, you may interest in Freesale SMAC.
Schematic of XBee S1 Rev.A is avaliable in the following document:
Hope these information can help you. I suffered a lot from these problems before.