I think that a 1990 Land Rover would not actually be called a Defender- instead it's a 'Ninety'. In that case, the engine is the 2.5 Turbo Diesel and it would have the LT77 gearbox.
The engine does have a rather bad repuatation for burning out pistons, but this is caused by abuse and poor servicing- yours seems to have been looked after well so you should be fine. Before starting the engine, heat up the glow plugs for 15 seconds. The engine should start quickly without large amounts of black smoke. A small puff of smoke and some blue smoke when it's cold is alright, but there should be none once you're on the move. Try the engine at lots of speeds on all gears and listen for rattles and knocks- the engine should be quite smooth, making the typical diesel 'tuktuktuktuktuktuktuktuk' noise. Once the engine has warmed up, check for oil leaks, especially around the turbo.
The gearbox can be quite stiff and notchy when cold, but should be fine once warm. There will be some whine coming up through the floor, but any very loud noises mean the gearbox will need work. There should be no grinding or crashing noises as you change gear. If there are the synchromesh needs work, but this is quite easily fixed. Try the Low Ratio and Diff Lock lever and check the Diff Lock light comes on when it should.
The bulkhead and chassis can both rust. Check the bulkhead for rust around the wing mirrors, below the bulkhead air vent flaps and at the bottom door hinge.
Check the chassis all over for rust holes. The rear cross-member is the worst place. It will rust from the outer ends inwards. The chassis outriggers are the next place to check, especially around the body mountings.
The steering should not be heavy and it should not have too much slack. The Land Rover you're looking at will probably have power-steering, so check this system for leaks and groaning noises (repair is expensive). The steering swivels at the ends of the front axles should be smooth and not rusty. They should not be leaking large amounts of oil either, though if they leak a bit they can be replaced.
Generally, I'd expect a 1990 Land Rover to have plenty of life left in it, and it should not have any big problems. A 7-seater version is a good choice- they're very practicle and fantastic off-road.
Good Luck,
Jack
