Here is what I would do:
First off, ask if the sellers would be willing to write into a contract that they will refund the purchase price or give full farm credit if she does mature into a problematic horse (as far as bite or going over height, or even fertility, depending on what you are paying).
If not, I would move on.
If so, I would look closely at both sire and dam, and any siblings or other related horses and see what their mouths look like. If all is in order there, then move ahead w/purchase as long as that overbite was not more than 1/4 tooth off at this point.
If you show her in halter, the overbite, if it does not correct itself, will put her out of the ribbons, and in breeding, myself, I would avoid the horse altogether, be it mare or stallion.
Liz M.