Things to consider:
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<LI>Warranty can only transfer once. If you buy it and transfer the warranty, your next buyer is out of luck. Unless you have an "in" with a Dodge dealer who'll play some games accommodate you.
<LI>A Viper with 39,000 miles is like a regular car with 150,000 miles. I would guess the average 96 Viper has about 10-15K miles. There's not a lot of interest in high-mileage Vipers when there are so many low-mileage garage queens and creampuffs available. In the present economy, I would think $40K is a fair price for a 96 Viper in good to execellent condition with 39K miles.
<LI>600 HP from headers and exhaust is sheer fantasy -- at least 100 HP too high. If those are the only mods, they won't add anything appreciable to the resale value.
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My advice: If you like the car and want it for your own car, then buy it and drive it (assuming you're satisfied with its condition). If you're just looking to turn a quick buck, I'd pass -- you will probably have to sit on it for quite a while before the right buyer comes along. Six months from now you might have trouble selling it for what you paid.