I very good rule of thumb write up.
http://www.horizonhobby.com/Explore/...ArticleID=1563
By it a 2lb 3d plane needs 300-400 watts. That should be 80% of max so I would suggest at least a 500 watt motor. You can always prop down to reduce the wattage if to much. Also to get 500-600 watts with less than 45 amps would probably require a 4s pack. A 3s pack is about 10 voltages under load so at 500 watts that would be 50 amps which is above the motor's rating. I have been fining on my setups than 3s is not giving me the motor max (80%) wattage output. I've been changing to 4s packs and reproping to get more power without overloading the motor on plane that have room and can handle the extra weight of the battery.
I hope I have not confused you to much. Selection is a black art not a science. Or find someone with the setup your looking at since real world information is the best data.