Why do you think mileage is irrelevant? I'm really put off by something with 100k or so.
Looking at any other s type on auto trader with mileage around 60k they are asking £3-4k
Tricky stuff car buying.
That car is worth around 2.5k, so I would offer him that and no more than 2.6k, although if its a really well looked after model i would personally be happy to pay 2.7k if I was keeping the car for a long time.
The used car market can be a minefield and its not always easy to find a car without a chequered history or something dodgy going on underneath!