Hi Sam.
By looking for a second hand 05/6 Mondeo, you are undoubtedly going to be walking a mine field blindfolded!!
For a start your going to have to weed out the ex taxis, and reps cars that have had the living daylights thrashed out of them. Then there's the ringers rebuilt from write offs.
Sorry for pouring water on your fire, but I'd rather tell it how it is, especially as you say your self that you are a novice.
I'd also steer clear of Vauxhall for the same reasons.
You will need to take a knowledgable person with you to get a good deal I'm afraid.
As for mileage, there should be no real reason to chuck out a 100k diesel so long as it has been serviced properly. Service history is the key words. But don't be fooled by pen'd entries in the service book, any fool can pass a service book round his mates to top up any missing service intervals.
I would definately go to a mainstream franchise dealer for peace of mind. Don't be afraid to ring the previous owner to varify the car. You'll be suprised at just threat of that phone call can defuse the bull from the salesman.
I once bought a Renault 25 with two high security locks fitted discreatly in the top of the boot lid by the aerofoil. The trader was saying the car had one private owner and the 75k miles was genuine. I had my suspisions. So after doing a PNC check via a mate of mine (interplod), I found the car had been owned by cartier the jeweler's, which explained the secure boot and the holes in the boot floor for the safe. I rang them and got hold of the fleet manager who could fax me the service record. The car had actually done 175'000 miles but had been fitted with a new engine at 90k due to a radiator hose going and cooking the motor. I still bought the car, but at half the price the trader wanted and with a new mot and a years tax thrown in to keep me quiet.
Just be carefull Sam, the traders are desperate for a sale these days.
Steve L.