If you are still planning on trying this on your own...

Go to www.spad.org, and look at the Debonair. Is it within your building capabilities? Are you very fastidious about measuring and making sure things are tight, correct, and level? There's a lot more to learn than how to fly. Just getting the engines to run correctly takes more of my club's flight training time than any other aspect of the hobby. It is very difficult for a beginner to "trim" a plane in flight (you sort of need three hands and two pairs of eyes), so you will need to build very carefully so little trim will be required. If you decide to go with the SPAD the folks at the Simple Plastic Airplane Design forum at www.rccombat.com are about the most helpful and friendly you'll find anywhere.

I'm not saying I'd recommend this, but I'm trying to answer your question. Yes, they make plastic planes that are very durable. If the Deb seems a bit beyond your building capabilities, another possibility is the U.S. Aircore kit. The advantage of the Aircore is that the kit is very complete, so you don't have to go all over tarnation to find parts, plus it is about the most rugged trainer that there is. You would have to work very hard to damage the Aircore in a hayfield (but you might need a plane finder to find it!).
(You are correct that you will destroy a balsa trainer in minutes if you try to learn how to fly on your own.) But it's still a kit, so you need to learn a lot about setting up control surfaces, and the throttle, etc.

So, if you practice for hours on your sim, and if you don't mind crashing over and over, and if you have a talent for this, you would not be the first person to learn on your own.