200 to 500 watts per pound in 70mm single, twin and 90mm single, planes 2.5 to 6 pounds.
I use 40-50C packs. You don't have to use WOT all the time, but it's nice to have.
My Rafale 70mm twin is 5.25 pounds and 2.25 kilowatts. 428 watts per pound.
My T-50 was 750 watts for roughly 3 pounds, but I've increased it to 1200 watts, for Speed.
The Rafe is fiberglass, the T-50 foam. I'm not going to be doing much maneuvering with the T-50, she'll break apart. Also the Rafe, which I have not flown, is set up for speed. At 2.25 KW and a 4.0 aH 14.8 volt battery (40-50C) the flight time Wide Open is 1.6 minutes at the outside.

Decide on a good fan unit. Decide on an RPM for the fan, the WeMoTec is 45,000 max. Then you can get the motor watts/kv rating and battery voltage.

The decision for the fan is based on the available thrust, and you have to scour the web for the figures, from YouTube, Extreme RC.au and Efflux RC etc.
www.pw-rc.com ( Philip Workshop ) is a good source for data too.

The available thrust from a given fan unit, will compare to the weight of the plane. Higher weight plane, and wing loading, the greater thrust to weight ratio.
Remember that scale jets are high wing loading, and may be a handfull for a beginner. So, foamies, with light wing loading a easier at first. EJF has a trainer too. 70mm. www.ejf.com