I made a simple device like this for a friend of mine to use on his model helicopter.
I was originally going to just use an op-amp as a voltage comparator with a reference voltage and light up and LED when the battery gets low. That would keep the component count and the weight down. We got a working circuit which was going to be re-done in surface mount parts to keep the weight as low as possible, but in the end decided not to make a PCB and instead look at doing a more complex AVR based system that could transmit telemetry too.
If you are just going to use stripboard or matrix board then an op-amp to scale the voltage connected to the XBee is probably the simplest solution. The Kill-a-Watt is more complex because it deals with AC voltage. Since you have more analogue inputs you could use a dual/quad op-amp and add some extra sensors (e.g. thermistors for motor temperature).