Absolutely do not buy and OTTB. Apart from the psychological issues over 60% of these retire unsound. Hesitate with Arabians. (don't yell at me I know there are good ones out there)
Take an experienced person with you.......repeat repeat repeat. Somebody good with beginners or people just starting out not joe next door who owned 1 horse 10 years ago.
Order of importance.
Soundness (does not have to be sound enough to do three day eventing to be a good first horse.
Make sure the horse has had beginner riders on it before. Some horses that have always been ridden by experienced, balanced riders do not take kindly to anything less.
Very rarely will a great first horse last you for long.
Do not be fooled by the we will grow up together thing. Very bad idea.
Accept the fact this horse is a teacher and if he is good you will have no problem finding him another great home to teach somebody else.