[secant0give]

I usually go for an ESC that is rated about 20A above what I need. I think Gazz has mentioned this a few times as well.

The reason being is that in most cases motors cool down quicker than ESCs
The motor is mostly right up there in the front with good air flow and easy to put a finger on to see how hot it is. Often an ESC is tucked away where it's got less air flow and it's harder to check if it's warm as well. A good ESC can be more costly than the motor, but not allways. Bear in mind that motor will only draw current to match the amout it's loaded. So you won't harm a motor that is rated at 20A with a 40A ESC unless you load the motor so that it's drawing more than 20A.

You need to match the prop to the motor to load it correctly. If you load it too much it will draw more current than it's rated for and over heat. If you load it less then you are not getting the full potential out of your system. This is where your Amp/Watt meter comes in very handy. It removes the guess work.

Voltage X Current (Amps) = Watts

So you can change the power (W) by changing the voltage ie adding an exta cell in series or changing the current draw by increasing or decreasing the load with a bigger or smaller prop.

As far as lipo packs go rule of thumb is max current should never exceed the C rating times the Capacity. ie 2200 mAh X 20C = Max current draw of 44 Amps

If you look at the Hobby king listing for motors, you'll often see in the reviews where people have posted test data for those particular motors with different combinations of props it's a very handy guide for motor selection. These often include static thrust which is very nice to know.

Kv roughly means revs per volt. A high KV motor will rev much harder than a low KV one
It's more efficient to use a big prop spinning slow than a small prop spinning fast.