Look at all the usual things that you can find on here from people asking similar questions to yourself. Spending the odd hour trawling threads on here will be time well spent.
The most important thing to do is to go and look at it as though you're buying a 12 year old car. Because you are! Have a good ratch underneath, the back especially for any more chassis work that may need doing soon and see what may be leaking out from the front. Check all the switches work, turn on the ignition, check all the dash lights come on then go out. Have a drive. They're no ferraris but it should motor along reasonably and pull from low revs to above 3000 smoothly.
Disco TD5's usually have great bodywork, one of their good points which can fool you into thinking "what a minter" but any problems there may be are likely to be beneath that skin!