The temperment scale is very subjective, as Sahara has mentioned. You are better off asking a lot of questions, watching the horse being ridden (if you're lucky and can have your coach or someone whos opinion you trust ride - all the better) and if you are confident you can handle the horse, riding it yourself.
For example I would say that my horse, for a beginner, because he is so sensitive would be an "8", but for an experienced rider I would put him at a "3" or "4" for the same reason (sensitive and easy to aid if you know what you're doing)... I hope that makes sense... So it's more important to be upfront with your skills and ask a lot of questions. Good luck!!
They say money doesn't buy happiness -- well happiness doesn't buy horses!