Should I be nervous buying 2001 Base with just 31K miles?

[Perfectlap]

On the face of it the S sounds like the better option of the two. The extra power is mighty useful on real world streets where regular cars today have more power than than back in 2001. The S engine also has some internal upgrades that didnt make their way into even the base Carrera engine.

Total mileage is not created equally. You might have one driver who used the car on long trips once or twice a week. While another used it more frequently but on very short trips that didnt allow the engine enough time to clear out. So you need a bit more background on the driver habits.
An oil analysis by blackstone labs would probably be a good thing given the low mileage, that could give you some insight into the care and maintenance.