What exactly, is the definition of a "Well qualified customer"?

Somebody who doesn't really need a credit to buy a car, that is, almost nobody.

a well qualified customer is a potential buyer who can meet or exeed the minimum qualifications to finance a vehicle. these qualifications are based on the lenders experience on the probability of the buyer making all payments without default. the credit rating does not have to be a certain score. because things like debt to income ratio can effect the probabilty of the loan being paid out. money down is not important unless you are trying to get to a specific payment range. the money down only lowers the amount financed reflecting the monthly payment.

A well qualified customer is one who has the demonstrated means to afford the product (in this case, an automobile), whether cash, credit, or purchase order.

Somewone with an excellent credit rating of at least 800.

Perfect credit

A clean credit rating and a stable bank account.

Someone with an empirica score of 730 or more on their credit bureau

That would be one with Good Credit. and a lot of cash for the down payment. With a trade in.

Someone with money in their pocket or extensive credit.

one who has a very high credit rating