You need to remember that 190Ds were much less expensive than 123 body cars and as such have generally led a much harder life. Expect it to not have been maintained the way it should be.
That price is very high, they're seeing it as a car that gets good mileage and figure it'll sell easily to some rube. It probably will but not that easily... Try not to pay more than about $4,000...
I've got an '85 2.2l 5spd manual, its reasonably peppy. I suspect an auto would be more like a manual shift 240D.
Anyway, listen carefully to the fan blower motor, mine made a bunch of noise until I oiled it. The wiper on mine is getting really slow too.
AC work on 201 body cars is supposed to be very expensive and at the price they're asking it dammed well better work.
The car should handle like its on RAILS compared to a 123 car but the rear suspension links wear and ruin the handling.
My car has power seats, I'd prefer manual, mine make a bunch of noise.
Make sure the injector hard lines have the plastic clippies that keep them from vibrating. If they vibrate too much they'll break. The lines are getting difficult to get, I needed one and it took nearly 2 weeks...

Theres probably more that I can't think of right now but that should get you started. Harp on it being an automatic which will lower the mileage and make it less fun to drive. Good luck!

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Sadly Benz-less