You should NEVER teach a horse to rear. It may look cool when professional display riders do it, but the key word is PROFESSIONAL. They are fully aware of the danger of the situation they are putting themselves in.
Also, the horses people train to rear can NEVER be sold. It is putting the buyer in danger to pass on a horse that could be potentially dangerous - and that's what it is. No reputable trainer, even the most famous, will ever teach anyone else's horse to rear, only their own, and they are aware they will have to keep those horses in safe hands who know how to deal with their kind of training forever.
The levade is a different matter - it's more an extreme form of collection than a rear, and takes years of high level dressage training to attain.
Go for a safe trick instead, one that wouldn't kill anyone if aids were misread or the horse passed into the hands of someone who didn't know about it. The worst case scenario of teaching your horse to do this? It gets sold, and then sold again, on and on and people discover its dangerous 'problem' of rearing, until eventually it ends up as a dangerous horse in a low end auction, and on a truck bound to Mexico. Harsh, but it happens every day.