Usual car buying rules apply, which you can find in many places on here,and on the 'net.
My own personal car buying rules....
Mercedes built all their cars on the same line,in the same factory,with the same components. The main differences between cars you are looking at now are the places they have been driven,and by whom.
GOOD CARS AND GOOD PEOPLE GO TOGETHER.
Start with the advert. is the spelling correct? Are the pictures good? You gotta be pretty thick to cut a car in half when taking a photo,and even thicker if you then use it.. If you can spell,use technology and a PC you are half way to being a suitable MB pilot...
What's in the background in the picture? A nice suburban semi with a gravel driveway and a mint E-Class in the background? Or an Avenger up on bricks. Tells far more than the waffle thats been printed. Do they mention veg oil? Run a mile if they do. Not a thing you do unless you've done a lot of research and reading most old Merc diesels now are like gold to Pikeys who think you can pull a burger van with your free fuel.. Run,Run like the wind!!
Old Merc diesels sound like they are broken. A distinct noise of a small lorry engine and very different to a modern HDi or Common rail. listen to a few to learn the sound and watch carefully...even though they are clattering away,they should be smooth and even,put your hand on the rocker cover and shut your eyes...you are feeling for a "miss", "stumble" or a rocking from side to side...infact a slight side to side movement is good as it tells you the hydraulic mounts are still functioning... Usual visual check for oil leaks,coolant stains,check the oil pressure gauge is at 3 when at 1500+ RPM, and above 1 at a hot idle at least. Check the heater is hot,and goes cold when the temp adjuster is turned,check it switches off with the key-these are all vaccum controlled. Leaks can be a pain...
Dip the oil, which will be black. It should be runny and wipe off easily though. if its thick and the stick is covered in carbon its been neglected. Coolant should be clear with no sludge and be a nice dark blue,green or even pink colour. Fuel filter should be clean and fairly shiny,a sign of recent servicing,and have a look down behind the inlet manifold,hopefully the glow plugs should be fairly shiny too,a nasty job to change if they've been in there a while...
Now's the time to look at any paperwork,reciepts etc and see if they back up what you've seen and heard. I generally disregard this as its so easy to forge or even buy off Ebay. I have a FMBSH for a W124 300E sportline in front of me at the moment,along with handbooks etc....I found it... I've never owned a 300e!! If I ever find a Beryl blue 1988 Sportline it can then be a 45,000 minter with a few hours work...
Paperwork backs up what you see, don't get fooled into seeing what the paperwork says.
P.S.
Another tip for an older Merc Diesel. The engines were fully encapsulated. The 190D was advertised in its early years as "The whispering Diesel!!" The under tray should go all the way back to the rear of the gearbox,and completely hide the underside of the fron of the car. the bonnet should have a thick covering on the underside too. If these are still here and in good condition this can only be a good sign.
I lost my undertray a few weeks ago to a stray log under the snow. I'm still upset about it.. Grrr.
