Which board to just send a simple serial command over RS232?

Hello - I'm brand new to Arduino and I need help selecting the right board for my project.

I think Arduino might work for a simple project I would like to build. I have a circuit that it usually off (no power or voltage.) When the circuit gets switched on, a control wire will be powered up with +12VDC. When the +12VDC appears on this control line, I need to send a short ASCII string (just a few bytes) to another device via an RS232 port. After that message is sent, nothing else needs to happen. The +12VDC might stay there for a while while the equipment is being used, but eventually will go away. Next time the +12VDC is present, the same thing should happen again. (The specifics here are that one piece of equipment is being used to send a "wake up" instruction to another piece of equipment. When power is removed from the first device, it will automatically power down the second device.)

Which Arduino board would be the right one to use for this? Can it somehow be powered from my +12VDC control line, or do I need a separate power supply for it? The +12VDC control line is limited to a maximum current draw of 105 mA, so if it needs more than that, I need a separate power supply for it, which is fine. And where can I find the specs on how to send the ASCII string at the proper parity, and number of data bits and stop bits? (I will use Serial.begin(9600) to get the needed baud rate. Are n, 8, 1 already the defaults for the other settings?)

Thanks in advance for any assistance or guidance.

Any Arduino board can do that.
I'd go with a promini, and FTDI Basic to download code into it (www.tinyosshop.com) and a RS232 board from www.nkcelectronics,com (female connector? male connector?) for the RS232 levels.

byte inputPin = 2; // 12V fed in thru 10K/5K  resistor divider to make 0/4V signal
void setup(){
Serial.begin(9600); // default of 8N1 = 1 start bit, 8 data bits, no parity, 1 stop bit
}
void loop(){
while (digitalRead(inputPin) == LOW){
// hang out here
}
// got a high!
Serial.println ("wake up");
while (digitalRead(inputPin) == HIGH){
// hang out here until goes low again
}
} // go to top of loop and wait for next high

Be sure to figure out what voltage levels you need for the serial connection.

Unfortunately, people constantly misuse "RS232" - RS232 implies +15 and -15v, while TTL serial (what comes out of most USB serial adapters, and what the UART pins are) is usually 5v (or 3.3v) and 0v. If it's using RS232 voltage levels, you need an adapter (normally you use something like a MAX232, which is a single-chip solution that generates the RS232 voltages and shifts the signals to/from those voltage levels).

It's not uncommon to see any sort of serial referred to as RS232, regardless of whether it's using RS232 or TTL voltages.

You need to use a resistor divider to knock the +12v signal from the input down to 5v so it won't damage the arduino (as noted above)

Code wise, I would keep track of what the value on the input pin was, and it'd look something like:

byte lastPinValue=0;

void loop() {
byte pinValue=digitalRead(INPUT_PIN);
if (pinValue != lastPinValue && pinValue) {
// do whatever you need to do when it goes high.
}
lastPinValue=pinValue;
}