In my mind a well-qualified buyer will depend on the vehicle and dealership (chrysler capital for chrysler, GM financial for GM's, etc). Typically It would be someone in the 740+ credit score range on auto-enhanced scores, with a certain amount of money down depending on the particular promotion at the time. Income typically doesn't have much of a factor in being "well-qualified" as long as your DTI/PTI limits are in line, which typically depends on the lenders. Most lender's like to stay around 10-15% PTI based on your Gross income, and under 50-55% overall DTI. Also you typically have to have some sort of auto loan experience because it's not unusual for a college student to have what I consider a "fake" 740+ credit score from deffered student loans and a 500 store credit card, with no real comparable credit.
Aside from credit score though, many national lender's don't strictly go off of credit score for placing customer's in tier's. For instance, Well's Fargo Dealer Services, may place a 720 credit score in a tier 2, while a 689 credit score could be a tier 0 or tier 1 because they have internal scoring systems that judge the ridge associated with that applicant based off of other factors than credit score.
