I have owned a couple of TD5 Disco's and just bought another one and have never had any issues with them and they have all been very very high mileage ones, the current one having over 200000 miles on clock(new engine though). look at the service history as you can get a good idea of its life and also if its got reciepts for work done.Dont be put off by high mileage, if its been well looked after and had a lot of the troublesome bits sorted it could save you money in the long run!
Check the underside of the vehicle for any excessive corrosion, as it seems that they skimped on the under body protection a bit at the factory!