I just don't trust very low mileage vehicles. All the seals need to be lubricated or they'll start to dry out. The rubber will still age (not as quickly as if it was in the elements, but it can dry/crack as well). I'd just be cautious about the car. I think with the "new" transmission in yours it'd last another 50-100k, but IMO a spare car is one of the best things to have lying around.

If possible find out when the miles occurred (sometimes I've seen them with 75k on them sitting for several years because the PO planned on fixing whatever ails it, like a RMS or headgasket, but eventually just decides to sell it) as it might tell you more about the car. If its some old couple who drive it to church and the grocery store, and have been for the last 15 years, then you might be OK. However, I just think a car with <5k miles/yr is guilty until proven innocent..

--
If you're not driving it "like its stolen," are you really driving?