Good for you for wanting to build. I would suggest you find a used 40 size engine. Look on ebay. I say this because a bigger plane is easier to fly and carries the weight of the radio gear better. Then build a kit or two to get the feel of what wood grade to use where needed, You can find almost new radios from folks wanting to upgrade their older stuff. There are a better selection of 40 sized kits. After a kit or two then look at a scratch build. Before you go to a lot of trouble to build a plane for granpa's motors make sure they run.

For a first scratch build look at the STIK series of planes. Great flying models and a good way to ease into scratch building. The Sweet Stik is one of the best models you can have and they are not a complicated build. Want harder? Build a Sig Senior Kadet. Its all sticks and is a higher skill level. Get a few plans builds under your belt, buy a model design book and scratch out your own design. Its a ton of fun to see your design moving around the sky. Good luck.