I am currently considering a 86-88 560SEL, 300SDL or 81-85 300SD. I hope the one I get is reliable. I think these typically are. As far as the reliability of Japanese cars, I think they are about the best- some of them anyway - even though my grandfathers 1990 Mazda 929 has 172K, but it has blown a head gasket before. I do not think Mitsubishis or the other Mazdas are all that great, but my aunts mother-in-law had a 95 Mirage that had 340K miles that finally died. She drove it on the highway a lot. Ironically, she has a 1993 Mercedes 190e that she bought in 1996, then drove it for a year until the brakes went out, then she decided to park it under pine trees, where it has set for years and is now green from moss. Sounds crazy to me.
Anyway- about the quality of Hondas/Toyotas- I currently have a 1993 Toyota Camry LE V6, with 211K miles and it still runs like brand new. There are NO electrical problems, no rattles or squeaks and the car is built very well, with high quality materials. My old 1991 Honda Accord LX 4dr had about 190K and still ran excellent and was also built with high quality materials. I have also owned other 80s model Honda Accords and Toyota Camrys in the past- some with well over 200K and all running well. In addition, my dad has a 88 Nissan truck with the 3.0L V6 and it has 250,000 and still runs fine. It has NEVER broken down the entire time he had it. He bought it in 99' with 200K from my uncle, who bought it brand new. It was never maintained, but still ran fine. It even still had the original spark plugs until two weeks ago.
I must say however, I think the quality of Hondas, Toyotas and Nissans have went down over the past few years however. I have heard of squeak and rattle problems with the new Camry's, Tundra's & Sienna's, as well as other problems with Hondas and Nissans. One good thing though, is that Honda and Toyota now also make some of the safest cars in the world today. Many of their cars earn excellent ratings in NHTSA's crash tests, as well as IIHS's "off-set" crash test.
I also had thought there was a lot of posts here about Mercedes problems, but it could be that this site has had more exposure than other sites. Most all car makes have boards similar to this on the internet. I am actually a member of several other forums very similar to this one. Some include, Honda, Toyota, Nissan, Acura and Lexus. I have not yet found a BMW site like this one. I guess all in all- any car can break down, but a Mercedes requires a little more care and maintainence than many other cars.