I don't think you build credit as a co-signer with your dad as main. Even with good credit used car loans have a higher % than new. You dad should be able to get a good rate though. Try plugging some numbers in the calculator here and see what comes up
Auto loan calculator, car loan payment calculator by Bankrate
E.g. 15,000 & 5% for 60 mo. = $283 ... it goes up about 7 dollars per full % point
At 5% the payments go up around $19.00 per $1000