Hi Daniel,
If you can extend another £2-3k, I would go for a lower mileage car. If you are only planning to drive 2-3000mi p.a. AND planning to NOT keep the car indefinitely, I would buy with the considerations of a future buyer (ie, always prefers lower miles, colour, spec, etc). The fact that you are not planning to keep the car for a long/indefinite time means that resale is going to be more important than just the low cost now. Low cost now, but come resale time, it's even more distressing not being able to FIND a buyer who is willing to buy a then-12-13 year old supercar (with associated costs of potential repairs) with over 100k on the clock.
If you can afford a 993 now, then spend the extra £2-3k and find the lowest mileage car possible. Imho, with each successive year on an older car, the lowest mileage will maintain values as older/higher mileage cars are written off unless a hobbyist wants to burn a LOT of cash rebuilding/rejuvenating/etc an old and tired car. I think the most desirable cars in a few years time will be low mileage 993TTs and S's.
I think there is a HUGE psychological barrier as 100k miles are approached/breached. Just my $0.02.
(btw, I went through exactly the same predicament as you and ended up going with a more pricey, but much lower mileage/right colour car. I'm glad I did as I see a LOT of higher mileage/tired 993s which simply aren't selling at dealers. The low mileage 993s, however, sell like hotcakes and always command a premium amongst the diminishing few who are looking for a 993)
Cheers -
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