before you do anything else, go visit
www.edmunds.com - it's a fantastic website with invaluable information.
specifically, it tells you the "tmv" (true market value) of all the cars. in other words, how much you
should pay for a fair price on the car.
is your dad going to finance with the car dealer? if so, i highly recommend he obtain financing somewhere else first. adding car dealer financing makes things much more complicated. as long as you have 'cash in hand', then all you have to negotiate is the price of the car.
i walked into the dealer, told them i know what their invoice is, what i'm willing to pay, and that i'm writing them a check. they freaked, but knew i had them.

of course, i ended up buying from a private-party, but oh well.