Flywheel -
If it sounds like a bag of spanners (more so than a derv normally does
), if it struggles to start (swarf from the flywheel gets into the starter) or the clutch squeals when open, even quietly (same as above for clutch release bearing).
Cost to fix should be £600 if you shop around. Its all about how it's driven - if the current owner has had the car for a while, get him to drive you in it to see how he drives - you dont want him to be labouring it in too low a gear and making rough changes.
Injectors -
Smoke on startup (preferably cold after its been sitting for a night) means an injector is dripping into the cylinder overnight. General smokey/rough running means one has gone - push it to the redline in second, if it goes into limp home mode with the glowplug light on then one's on its way out. Injectors can be had new (genuine delphi ones) for £100 a peice or £350 for 4 and its an hour's job to fix, so max £400 to sort.
Anyone who tells you the problems will cost £1k upwards is going to main dealers, ignore them. If you get an annoying rattle/buzzing when acellerating around 2000rpm, its a very simple fix involving a cloth
Mine's done 55k miles, bought it at 36k. Had to do the flywheel at 50k but the previous owner was a woman who couldnt drive properly
No injector issues or any sign of them at all - if the car hasnt been trashed then these should last well past 150k miles. If you're lucky the flywheel will have already been done on that one as the new ones are apparently tougher.