Whether the price of the vehicle should change or the buyer/seller should pay for the repairs depends entirely on how it is currently priced!
Is it already priced low because the owner knows it's in rough shape? If so, then it's already priced correctly and you get what you pay for.
Is it priced like a mint condition, fully maintained car? If so, the price needs to come down or the seller needs to do some work on it first unless he is just waiting to see if someone who doesn't know any better will buy it.
Personally, I'd be more concerned about the fact that the vehicle seems to have been poorly maintained than about haggling over who should pay for what. Several of the items you mentioned are not terribly expensive to fix, so why were they all ignored? What else has been ignored? Did you do a compression/leakdown test? Does the owner have any service records to show you?
The timing belt is the only single expensive item, and until it breaks it doesn't hurt anything. But again, we don't know if the need for timing belt change has already been factored into the asking price for the car.