I would totally ignore mileage as a guide to the condition of a car, lack of use is often far worse than high mileage, if a car isn't reguarly warmed up and run everything will soon start to seize up and prolonged cold through winter doesn't do wonders for anything. It is also probably fair to say that drivers who don't drive a lot probably aren't as good drivers and probably aren't just cruising up and down motorways, which is about as light work as cars can get.
The first thing I would check is tyres, they can tell you everything about a car. If they're underinflated it is a pretty sure sign that the car has been owned by a moron who simply doesn't care or have a clue how to look after his car, walk away unless you are into £100 territory.
If the tyres are boardering on illegal it tells you the previous owner was almost certainly on a tight budget, and may well have made cut backs elsewhere.
If the tyres aren't a matching set you know you're probably dealing in £15 a tyre part worn land, nothing wrong with that on cheapish cars. I do it all the time and so long as you insist on getting premium brands it is an effective, cheap way of getting decent rubber on your motor. If you're buying a 911 or M3, however, walk away from part worn/budget tyres because the previous owner obviously couldn't afford to run it.
Then there are the cars with their original set of tyres on, if the tyres look prehistoric then they really need to be binned. Also a sure sign of either a very low mileage car, which is going to have lack of use problems.
If it has got a matching set of decent tyres you know you're probably dealing with a well looked after car.
I would also avoid dirty cars, if the seller can't be bothered to clean it when they're trying to sell it then they obviously don't care for it.