Tomorrow I will be going and likely buying a 2006 Prius. It has 190,000 miles overall but the majority of the miles are around town miles. I would be surprised if the engine itself has more than 120000 miles. The owner has receipts showing a brand new, not rebuilt or refurbished battery was installed back in May of this year. All other aspects of the vehicle are excellent. Paint is flawless. Interior is above average for its age. Suspension and steering are excellent. The engine doesn't leak any oil. How much would one expect this to extend the life of the car if properly maintained?
As little as possible. Take a look at kbb.com to get a guesstimate. Also look at local craigslist ads. Is this a new battery installed by Toyota?
If a new battery and installed by a Toyota Dealership, you can enter the VIN on Toyota's website and it will list that as a service performed on the vehicle along with any other dealership service(s) performed on the vehicle.
The paint on a 12 year old prius should have some fading, especially on the rear spoiler. If everything is perfect, I would suspect the car has been painted before. If the seller is a small dealer, I would check everything out very carefully. At 190k miles I would expect the car would need some repairs very soon. These engines although doesn't leak much oil, they often do burn oil at high mileage. So you'll have to keep an eye on it
