I have a 7 year old 3.0D and very few problems.However,I work at the dealer and I can say that some of your in warranty issues are not common ,but then again as bigwave255 said thats what warranty is for.Count yourself lucky that these didnt occur outside the warranty!
You were very unlucky to have to fork out for a new windscreen because of the light sensor failure.
Coming to the issues you have been told you now have ,I need to ask you if you car has been having problems cranking and starting problems and if you have white smoke with an unburnt raw diesel smell when you start the car?
Does your car start first time?
I ask this because ,if they want to replace your starter motor and your glow plugs ,these are the obvious symptons you should have !If you dont have these,then you have to ask why they are wanting to change these parts?
Do you have an injector error code registered?Did you notice any performance drop in your car lately or excessive smoke when car is hot?
In my experience,with your mileage and age,its far too soon to be replacing start motor,glow plugs and injector....but you have to,then you know that these wont be a problem for the coming years.
Had you had any major engine,transmission issues ,then I would say,sell the car but if you like the car,just get a second opinion on the issues the dealer recently pointed out before you authorize the work and if its confirmed that they need replacing ,I would do them and enjoy the car for many more years.