UPDATE
I don't have the car YET, but it's about to happen.
Both my dad and I have been busy the last month, so we took our time working this out but the end result is I'm going to buy his Boxster.
It was very important to make sure we both felt good about the deal - neither side wants to feel taken advantage or - it's family! So we tried to work something out that was a win/win for both of us.
End result - Price = $13,500. But he carries a loan and only charges me 3% interest. it comes out to roughly $250/month which is dirt cheap in my opinion. As it's inter family - No sales tax.
I only need to budget for repairs and tires.
I am probably going to get a PPI - even though I know the history of the car. I drove it about a month ago, and it was making a hissing sound - like air leaking from an inner tube/tire when you gave it gas. I could only hear it with the top down when I was parking - driving really slow but on the gas a little bit. At speed - there was too much noise to hear. I wonder if it's an exhaust leak or vacuum leak. I'd like to find out what that is, and maybe jsut get it fixed while it's there. Then it would be nice if they would just look it over and tell me that there is nothing horribly wrong that someone like ME would not see, but a mechanic who knows what to look for WOULD see. The car has been serviced at the same dealer - so not only will they have the car, but they will know their own maintenance history on the car too.
I should have the car in the next week or two once we get all the details worked out.
Now I just have to drive it, enjoy it, and not worry too much about possible motor failures...

I am not going to buy the warranty. $5000 is too much. maybe there is some cheaper warranty that would NOT cover small stuff and ONLY cover the mega huge stuff? we'll see.
thanks to the people here on the forum for all the help and advice. I know I am going to love this thing. I have driven a lot of cars, and the boxster has to be THE most fun one ever.