the liquid cash you're sitting on that you can fuggin pick up without a prior arrangement is your budget. not what you can maybe "borrow from the bank".
if you don't have that loot stacked and racked, looking for a car is a damn fool's errand. if you gotta ask mommy for the money by the time you get your bag together that deal is gonna be gone, or if it's still there you've just lost any kind of negotiation.