How much should my initial offer be for this car?
According to NADA, the "clean" trade-in value on this car is $8,825, while a "clean" retail sale would go for $10,650.
However, with the problems you list, it's far from "clean." Average trade-in on this vehicle is listed at $7,975. NADA's definition of "average" is a vehicle that might need significant reconditioning before it would be ready for "clean" retail sale.
Knowing what we know about the economy and how slowly cars are moving these days, I wouldn't be afraid to offer to guy $8,000 or walk away. If you're willing to go higher than that, offer him $8,500 and tell him he would get a much worse offer from a dealership if he were to try trading it in for a new car-- reminding him of the condition of the tires and windshield.