If your thinking of a brushless motor, then now is the time. Because you glue the motor down on the UHU, I think it maybe a little hard to remove afterwards, so it's probably best to put a motor in now which your going to stick with. There's plenty of choice regarding brushless motors. I would look for something with a KV rating of about 2000. I could name a few, but it all depends on whether you can get them in Holland. Its an idea to have a look around, then if you see one but your not sure, let us know. What battery will you be using ?
As far as reinforcement, there isn't anything you really need to do. I never bothered with the two wing wheels that come in the kit. Unless you take of from a silky smooth runway they'll be no use. Just run a little tape on the underside to protect it on landing.
Hope it helps,