Look for dings on the rims, exhaust outlets, easy to do on the Viper, especially on right hand turns, hit curbs, ding wheels and or exhaust. If you get the serial number from the owner, you can go to a good local Dodge dealership, and they can run the records up for you on the car to see if it has had proper maintenance, etc, service records, etc. The dealership will be glad to do this for you, especially if they have a Viper Tech on board, because you will probably take the car there for service! It doesn't hurt if you know somebody at a Dodge dealership either, as they can run the numbers on it for you. Get it up on a lift at the Dodge dealership (don't go somewhere else as they may not know how to properly lif the car, so an under car inspection can be made.) Check the underside of the heads with a strong shop light to see if there are coolant tracks or oil drips. Check the tranny for leaks and the rear end too. A Viper Tech will look for all these things, and more.
Does the car have any warranty left on it? Or can one be obtained? (Dodge Dealer can answer this one for you). How do the tires look, still have a lot of tread left (tires are expensive for this car), about 1200 bucks and up for a set of 4, and fronts are smaller than rear, so rears are more expensive.
Don't let all this scare you, if you find a good car, you will truly enjoy its performance, it will continue to surprise you with its power and torque! Vipers are amazing cars!