Hi tlw85,
As Richard says, a lot depends on how the car has been treated since new. If a car's been thrashed by a speed demon who didn't bother much with servicing then it can be a wreck by 100,000 miles. But if the car's been well treated and respected then that mileage is nothing really - especially on a diesel.
I've had my 1984 VW Golf for 21 years now and it's done 237,000+ on the original engine. We drove to Gibraltar and southern Spain in 2006 and around France this May. The car is petrol but it still sails through MoT's and is sweet as a nut. That's because I've always kept it fully serviced and changed the oil and filter every 2,000 miles. That's the key.
You should be able to get a rough indication of the condition of your car with some basic checks. If the engine is lumpy and smoky when driving or revving then it might need some work. If it ticks over sweetly and the exhaust is clean then there won't be too much wrong - especially if it's recently passed an MoT.
The condition of the body and running gear also depend on how the car's been cared for. You should get an idea of the condition with some basic checks underneath but you should have done that before you bought the car. If the car has recently passed an MoT then it won't be too bad. The tests are strict now and almost all the 'old bangers' were weeded out years ago.
If your car is good then keep it regularly serviced and the oil and filter changed frequently. It''s cheap and the engines love it. Try to do the work yourself too. It's expensive to pay someone else and the only person who really cares and does a decent job on your car - is you!