I agree with phread59 on all points except two.

2) Use medium balsa for webs. I wouldn't use contest balsa here.

I would also not use polyurethane to join wing sheets. The foamed part sands easily, but the part that soaks into the wood doesn't. Use regular airplane glue (Ambroid) and wipe down the seam with acetone while it's still wet. It dries faster than polyurethane and is much easier to sand.