Another thing to consider - price - a 175K mile, £3K Mk5 GTi is fundamentally the same as a 20K mile, £7K MK5 GTi. Sure, one is a bit more worn than the other but they are fundamentally the same car and one is less than half the price of the other - is it worth twice the price for the extra miles?
Resale value - if you're keeping the car for 5 years / till it dies, the £3K car above is going to be worth 1K with 250K miles on it. The £7K one will be worth £3K, so you've lost £2K or £4K respectively. Which works best for you?