the watts/lb is something more acurate than you think.
100W/lb +- 25% is a good rule. You can't get wrong using this.
The second point is , once you know the watts required, to choose the right motor . That's more diffcicult as you need to find a motor with a good efficiency % at the level of power required. You need to choose between brushed, brushless, outrunner, gb, prop size.....
there you have to check things such as motor Kv, amps,....or you can ask for prior experience of others to pick the right combo motor/prop.
A whattmeter or equivalent is useful to determine amps and watts used.