when you see the car - it should be dead cold, if not then the owner is trying to hide something. A dead cold diesel should start on its first preglow time - i.e. glow light goes out and the engine starts quickly with a puff of black smoke. (whitish diesel smelling smoke is a sign of extreme cold weather) warm it up and drive it around it should not have sluggish or dead engine response - look for excessive burnt fuel exhaust. (black).
once warm pop the bonnet and check for induction noise or air pumping out of the air filter (bad timing + valve clearances), check engine knock noise it should be even not the occasional added knock - and when revved the noise should diminish - it should only knock when idling. Also note if the owner has been using coolant or plain water - are the rad fans direct running or auto - ask if the owner has ever changed the headgasket.
You can also check its compression a cheap way if its manual transmission - park it backwards on a slope and apply 4th gear with engine off - let go of the brakes - you should feel it stop on every cylinder compression stroke. If yes, its a solid engine - if no its compression is a bit low (this is also a good time to check if the original glow timer works - a hot engine glow time would be very small or next to nothing)
about owning it - filters are the best life line, and use some branded industrial filters in it - (they work far better and you can buy them easily) - secondly your issues can be glow plugs burning out if used excessively - and you needing to buy a beefy battery with correct mixture quality for it to start on first key (I cant emphasize enough on pure components to make the acid mixture and put in a quality battery - with good battery cables etc.) Any diesel requires solid starter shoves to overcome the high compression bumps
Check for engine blow at idle with the oil cap sitting on the cover but not tight - if its just plopping about - with no excessive smoke - now rev the engine to twice the idle speed i.e. about 1800 revs the oil cap should be steady now, if its blowing harder - but if it is blowing hard - your compression test would have failed too.
Finally - the timing belts are tricky on this engine as there are no marks on the cylinder head to line the pulley with, the belt is marked itself for alignment, likewise for the injection pump.