Hope you are scratch building! those kits are a gold mine, everything I've read says to copy the plans and tuck the kit away. I was watching a Cleveland Models b-17 on ebay a while ago, they go for some insane $$
as an example….
http://www.oldmodelkits.com/index.php?detail=13327
I've settled into using E-Flite power 15's for my model. They can put out 475 Watts of power, so 4 of those is 1900 watts, if I hit 11lbs which is my target that is 172 Watts/lb which is way more than I'll need. Way way more. way way way way… you get the idea. I could have probably gotten by with the power 10s even on this plane at 11 lbs,
To give you an idea, as the plane sits right now the body weighs 5.5lbs I believe.. its been a while since I measured. the question for you is how much is the buildup going to weigh, as the rest of it.. radio, landing gear, finishing supplies, engines, batteries etc are all going to more or less weigh the same.
The smaller your motors, the less battery you need, the less the plane weighs… ( the smaller the motors you need… ) its a bit of a circle, but the battery weight is significant in a plane like this and is going to be the main challenge.