You can tell if it's a new motor easy without taking apart the gun. Just take it out the handle and reconnect it so its out the gun and try it. You can test the torque on it by lightly pushing it into soft wood and seeing if it chews the wood up, just make sure the teeth are cleaned before sticking it back in the gun.
What do you mean fried the wires? Are you using the correct size wires or did you rewire it wrong and the wires hit the moving gears? Sounds like you've already had a go which is a good start. I'd say don't give up, these things aren't rocket science. If it's just replacing old with new, have faith, you can do it! And we're here to help