ORIGINAL: B.L.E.
"The Kv rating and amp rating are the specs you need to match."
As he said!
You can trade off almost everything else... e.g. higher cost for lower weight, lower cost for a bit more weight, different voltages provided the AMP draw is about the same, etc.
A power meter becomes a MUST if you go this route. It's the only way to establish if that new prop you want to put on is going to damage the batteries, ESC or motor.
You can purchase some very cheap motors, which while NOT as efficient as the better motors, do quite well.
I just finished putting in one of these into an E-Flite Diamante .25 which I will be running using a 4S LiPo.
[link=http://www.nitroplanes.com/mo258brmo.html]Click me![/link]
Thus far I'm quite please with the draw, balance and performance of the motor - BUT - it's almost 2 oz heavier than the E-Flite equivalent.
But I'll sacrifice a bit of weight...