Here's your problem: A 1986 300E could be up to 18 years old. Find me an 18 year old ANYTHING that won't be coming to the end of it's life. Not many cars live that long, period.
So, all the 86-88 cars have been sitting on springs for years. Bushings have all aged. Shocks will have been shot for a while, if they haven't been renewed. Transmissions have shifted tens of thousands of times, maybe hundreds of thousands of times.
Okay, now that we've scared you, here's the reality: An early 300E can still be a great car, but make sure you budget the right amount to keep that age of car running, otherwise you'll be frustrated and blame Mercedes for making an inferior product, when the truth is that it's a very old car.
The problem with buying older ones where I live is the price. I could buy a 91-92 300E for about $15K, and 1986 owners all seem to be asking $10-12K. Those 1986 300E's need to be $3000, not $10,000. I've seen some for sale for months before the owners give up.
Unless abused, the bottom end of the engine will outlive the rest of the car. The M103 is overbuilt, especially considering it's "modest" (high in it's day) 177 ponies. Top ends are weaker, but still last many, many miles. If you find a car with a recent top end (done by a good shop) the engine will probably outlast the rest of the stuff on the car.
The transmission is very durable, but given the age/mileages of these models, you may be in for a rebuild if the PO had not already done so. I have a friend that is intent on keeping his 87 300E forever, so he bought a Mercedes rebuilt unit for his car. Had the rear main seal done at the same time, and suddenly his driveline is renewed. He's diving into suspension bits now, and this will run some serious damage on his wallet. Is he nuts to put $6-7K into a car worth maybe $5000? Sure. But, so what? He'l keep the car for years, and probably come out ahead. I wouldn't do it, but that's just me.
If you can live without the AC working, you might be able to get a stral on a car being sold by a frustrated owner facing a high AC bill.
Go shopping, and see if you can find a much later model for not much more money. With MB's this is often the case.
1998 C230 "Black Betty"