He is right about only about 5000 made. The rare one is the long wheelbase version (not 300SEL, but 300SE Lang) though. The prices quoted are silly though.
The valuable one is the 300SE Cabriolet and to a lesser extent coupe. Even the long wheelbase four door is not even close to that value.
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I must admit my surprise that a 300SE Coupe sold on Car Sales for the amount quoted. If that is correct, the market has quadrupled over the last few years. I really don't understand it as the cars are really not that good - style wise fine, but in that regard, any Convertible version looks better. Our own C107 has a beautifully kept 250SE Convertible, and it is in exceptional condition.
Regarding the car advertised - it is the standard wheelbase version, and obviously in very poor condition. This is probably a stab in the dark, but it suspiciously looks like a car that I once looked at with a view to purchase about forty five years ago. The old chap wanted $500.00 for it, but when I offered $300.00 he got annoyed and took away a glass of water that my son had asked for as, obviously, he was thirsty. I am digressing. He quotes $80K. top to bottom restoration. I guess you could do it for close to that figure, but I would want to be much, much younger than I am now, and I would also want to have a lot more liquid funds to undertake the restoration.
The car that I owned, viz. the "Lang" version, I bought for $2300.00 back in about 1978, maybe a bit before that. It was a great family car. Huge boot, self levelling, roomy interior, and an uncanny engine with typical operational noise on account of its all alloy construction, but a surprisingly fast cruiser on the open road. 90 miles an hour was child's play. The engine itself is probably one of Mercedes' most expensive engines to rebuild, and always was. Gearbox nothing special, very similar to all of the "60s gearboxes. Interior about $25K. from go to whoa but it is all accounted for in the $80K. price quoted for restoration.
I think the preferable model would be the next W109 300SEL of 1967/1968 vintage and I know where there is one (not for sale at this stage) awaiting restoration. In many respects, it is a much sounder car than the one advertised. Again, a lot of work if you have the time, patience and money. I doubt that its present owner will ever get around to restoring it, but I must not put the car before the horse. I will probably be able to expand further on this car and the various myriads I can come up with, and I consider it to be a quite fascinating subject. I even have some pics. of the 300SE I owned for that period of seven years. Regards Styria