A very long time ago a good friend and petrolhead "explained" to a car & bike mad teenager (guess who
) that the only way to get high mileages on something was on Motorways & major roads. Cruising, dude on decent(ish) tarmac
There was more wear on the "one careful lady owner" car that went a couple of miles to the shops and back once a week all around town with stop/start traffic and never getting properly warm.
Oh, and in general, bigger engines are less stressed than smaller ones 'cus they work less for similar effect.
I have no personal experience of stellar mileage 2.5's but provided it's had regular oil changes and been obviously looked after I don't see what would make it implode the minute you buy it.
Hope it works out for you.