Wow - a question on Benzworld where I think I can actually help.
For plastic and vinyl areas - like the dashboard, for instance - the answer kind of depends on what you want. Do you like shiny? Do you prefer your plastics to look darker, with maybe a bit of sheen to them, but not necessarily glistening? Or would you rather use something that leaves no trace at all? I have tried many, many detailing products over the years. Hundreds of them, probably including a dozen or more interior protectants. Here's what I like.
The most versatile, and best bang for the buck, is Vinylex. It's made by the same people who make Lexol leather products. It's in a light blue spray bottle and you can buy it at most major auto parts stores. It'll go on shiny if you're heavy handed with it. Or, you can buff it off after it has set up, and leave more of a satin look. It's even good on tires, weatherstripping, and under the hood.
A good competitor that's a bit harder to find locally is Autoglym Vinyl and Rubber Care. It behaves similarly to Vinylex, but smells like lime life savers. You might have to buy it online, but I've seen it at Pep Boys.
Prefer something with no "look" to it at all? Try Einszett Cockpit Premium. You'll need to order it online, but it's worth getting. All of Einszett's interior products smell great and work very well. It's a mild cleaner with some small amount of UV protection in it. Not as much as Vinylex or Autoglym VRC, but it's "something."
Recently, though, I've found something I like even more than any of them. Mercedes' own "Cabin Care" aerosol is my hands down new favorite. It splits the difference between Vinylex and Einszett - it'll leave a dry, black Mercedes dashboard looking much more dark and new, with NO gloss to it at all. It's really excellent stuff. I'll be buying a LOT more of it. Worked great on my girlfriend's Camry today, too. I think I bought it for about $10/can at the dealer.
Leather is a whole other topic, obviously. The Mercedes foaming spray seems quite good as a one-step cleaner and conditioner, but, I think there are better pure cleaners and conditioners available. Surf City Garage makes a product called "Dash Away" that you can use to clean lightly soiled leather, as well as any other interior area. It's great, and you can get it at Pep Boys.
Many folks seem to swear by Leatherique, but I haven't tried it on heavily soiled leather. I use pretty aggressive stuff on very dirty leather, made by a local company here in NC. Strong stuff, and not for everybody or every car, but man does it clean.
Finally: when cleaning any part of a car, always use the least aggressive thing that will work. Often, people assume that stronger is better, and it's not. Leather can be a little delicate, especially on an older car, so try mild stuff first for cleaning. I've even used a mixture of 10:1 water:Woolite on leather, with pretty good results. You might even be surprised what straight water can clean.
And don't use a Magic Eraser. They have their place, but leather isn't it.
Hope that helps!