The air filter is the first place I look as just trying to establish if past owner cared the slightest about maintenance or car at all. Unfortunatly most my aquisitions have had filthy air cleaners. Since this is absolutly the minumin effort to check and replace on a diesel and the results so common and generally known if dirty when i find this it means I will have to check everything of a maintenance nature out sooner rather than after awhile when I get time. Your case sounds simular. You almost know now that the other more involved things have probably not been done either. Some people feel a diesel needs nothing other than fuel filter changes no matter how long they drive it if even that. One thing to check pretty quickly is timing chain condition if the records you have show not changed at recommended interval as if it decides to part company the least of your worries will be the items mentioned in your post. The timing chain will still need checked though as if car has not had it's regular oil changes the chain is the most wear sensitive component with dirty oil inside the block. Think your engine is touchy about overheating as well so check and replace all hoses in cooling system that are even a little suspect. Your other items are most likely modest in comparison. Check belt or belts very carefully as well. Make sure car does not have green antifreeze, Look and see if brake fluid clean or dark. All will give indications and in some ways may make your other complaints easier to rectify as they will almost be just general maintenance items as well. This site will easily get you up to speed if you read the posts pertaining to your type engine. If I purchased a 603 type engine would not want to loose it just from lack of general maintenance. We have never met but there are one or two gentlemen on this forum that service mercedes independently in the detroit area and it might be cost effective to let them have a look if you either do not feel up to it or have the time. From their help freely given on this site plus the generally balanced involvement they display I do not need a recomendation as it is about as good as it gets. They will take the systamatic approach to your engine that you may lack plus the equipment or experience to do. Their experience and integrity might also save you a bundle down the road. Wish there was somebody like that in my area but there just is not. Just a few thoughts and my opinions. Oh yes, welcome to the forum it's by far the best one I have had the pleasure of experiencing by far. How they tollerate me is beyond my comprehension.