The ones with the best detail, in and out in 1/43 are the Paul's Model Art and Minichamps. They are sold on mounted plastic stands inside a glassine case, and are thus easy to keep clean.
They do indeed have them in W115, W123,W126 and later. You can't find the older ones in MB dealers part depts, anymore, but they are available on www.ebay.de
(Germany). Click on modellautos, then try Minichamps or Paul's
Model Art, and click in the model series you are looking for to get an idea of what's available. Herpa is another maker who makes them (Mercedes) in 1/43 scale in a glassine box also, but they are cheaper, and the detail is not quite as good, I picked up a few of theirs (W123 and W124 coupe etc.) Lots of collectors and model car shops exist for these in Germany, and I have a couple of dozen I have purchased this way, one by one. There is even a German website that exists SOLELY for collectors of Mercedes Benz (only) model cars, I can find it for you if you want.
The German ebay sellers know English, pack VERY carefully, are fair honest and communicative. I have NEVER had a problem yet. Just email them first and ask them how much shipping and insurance could cost, and how to pay if you win the auction. (I ususally paid by sending US Dollars cash in the mail, or by paypal)
Some of the ebay sellers would be happy to sell to you directly, at a price given by them outside of ebay.
Some models are rather rare and not for sale too often; Minichamps sometimes made "special" runs with very few built. For example the F1 safety car, or one that I bought, a W210 sedan "Mary Kay" cosmetics edition, it was ALL PINK!!!
I have never seen one of those again. I also bought a "POLIZEI" edition of the W123 sedan and estate wagon, it was a model of the autobahn police car. They now make also, the W115 sedan of the Polizei, and of a Taxi for the W123. They just issued a 560SEC coupe, in July, which you can get in champagne, gray or dark blue. I recently just bought a dark blue one and the detail was EXCELLENT!
For the older models, the Pontons, and the Heckflosse, these were made by the Predecessor of Minichamps and Paul's model art, the maker was Vitesse. These also have EXCELLENT detailing on them, and so I would highly recommend them, but they are difficult to locate on ebay, but occasionally come up. Some of them from Vitesse are sold with their own diorama, like a Taxi from Spain.
It can get addictive, so be careful. The prices vary according to demand, and Luck (if bought on ebay) but for example, the 560SEC 1/43 W126 Minichamps coupes prices were from about 27-32 Euros, shipping extra. Mine was airmailed to me, I think it cost about $55 (USD) when all was paid. Not especially cheap, but these are impossible to find in the USA.
I recommend highly the Paul's Model Art, Minichamps, and Vitesse for the 1/43.
Their quality is known to be the best and the detailing also. Ebay Germany seems to be the best place to look at what is for sale, and for buying them.
Forget the 1/18" scale, they are cheaper sometimes, but generally the detailing is not actually as good as the best 1/43" models.
The address for Minichamps is:
Paul's Model Art GMBH & Co. KG
P.O. Box 50 05 04
is their website. I understand they issue a yearly catalogue, and they of course make 1/43" models of many other makes, such as Volvo, Opel, Ford and such. I have a couple of Volvos from them and the detailing is perfect.
I hope this helps you!
Good luck in starting your collection, and with the 1/1" scale W124 230 saloon if you get it also