Did you have a mechanic of your own choosing give this vehicle a thorough checking prior to buying it?
Have you checked the level, color, and odor of the transmission fluid?
Do you have the maintenance records from the previous owner(s)?
Personally, I would be worried about a vehicle that has been driven–on average–only about 6,000 miles per year for 15 years, as that could represent purely local driving patterns…which are far harder on a vehicle than highway driving is. In a situation of purely local driving patterns, the previous owner(s) should have been using the Severe Service maintenance schedule, rather than the “regular” maintenance schedule. Can you verify that this was done?
Please go outside, check the Owner’s Manual for the correct procedure to use when checking the trans fluid, and then report back to us on the color, level, and odor of the fluid. Also, please answer the questions that I posed, above.