From what I understand what decides your APR is your credit score (they use a formula that is specific to car buying if using dealer), the lender and the dealer if using dealer financing. The dealer may add a percentage to the interest rate and they get a cut of the profit. I don't think a bank would lend you for a $80K car even with that down payment based on income and banks tend to be annoying to the self-employed, specially if the business is new. Your best bet is the dealer IMO. If you find unfavorable terms or something isn't as agreed on F&I you can still walk away. You are not obligated to buy from the dealer unless you sign the paperwork.