We just took over the care of a 1985 300D. It was my Mother-In-Law's car for more than 20 years. I had driven it off-and-on through the years, and always thought it was a great car. We are delighted and very grateful to have it.
First of all, they are amazing cars. The design and engineering is remarkable. Also, the aesthetic is certainly unique. I don't know how long you have been driving, but, if you have never owned a Mercedes (or any car from that era really) you should probably spend as much time with one and do your research as much as you can before you decide to buy one.
They just don't behave like modern cars. They represent some of the best in automotive engineering FROM 30 YEARS AGO!!
The handling, acceleration and braking characteristics is going to be very new to you if you have never been around one. Further, little things like lighting, climate control, sound dampening and just general creature comforts have come a long way in the last 30 years.
Also, despite the best in design and engineering, time takes it's toll on everything. Our car is the last year of the 300D. It has close to 200K on it and the motor runs like a sewing machine. Remarkably, the automatic transmission still shifts well and seems solid. Everything still works. The A/C, the climate control unit, all the interior and exterior lighting, the stereo (including CASSETTE!!!) "power" locks, cruise control. It's amazing.
But, there are countless knit-picking issues that you have to deal with. Door seals failing. Fuel lines weeping. Almost all of the electrical switches are touchy. I have removed and cleaned several of them already. others need to be replaced if I can find them. Most of all of the electrical harness is becoming brittle. It will be an issue in the future I am certain.
The plastics are becoming brittle. I snapped the exterior hood pull the first day I had it. The grill insert is broken is several places and will continue to do so. UV radiation and time don't mix well with plastics.
Of course, everything can be fixed. I continue to be impressed with the availability of parts for a car nearly 30 years old. And, I LOVE, LOVE, LOVE the car. I would drive it all the time if I could. I love the nostalgia, classic design and general uniqueness of the car. I reminds me of some of the cars I owned as a kid. And, I enjoy wrenching on mechanical things. I also have a Harley Davidson and LOVE it as well for many of the same reasons.
A 300D is not going to be a care-free daily driver in any way.
I certainly don't want to discourage you. But, you should have perspective.