Best sensor for measuring a Punch

The cheapest option would be to use a very simple transmitter like the one found here.

The problem is you'd still need to read the sensor somehow to be able to send the data through the transmitter but that could be accomplished with a microcontroller that is much cheaper than a full-fledged Arduino. Each sensor would use a wireless transmitter to talk to a base (Arduino-based) with a receiver that would then act as the web server. That might be a bit of a daunting task if you're just starting out.

One simple solution might be to watch an output from your receiver. For example if there is no throttle signal because you have landed the copter, then activate your sounder. You might not need an accelerometer if you can reuse a normal signal for the task. If the copter is not powered for flight, it's either on the ground or soon will be.

After seeing at the reed switch idea below, I got another idea that is a combination of mine and the reed. Place the reed switch on the motor power lead, there is likely to be enough magnetic force to actuate the reed switch. Use that to activate your sounder. Worse case you would wrap a turn of the power lead on the reed. I used this trick on an old car with a bad ignition relay, the reed wrapped on the starter lead would close to bypass the distributor voltage drop resistor. Work like a hose.. lol.