Sevillian:
The ONLY leather cleaner and protection kit specifically developed (marketed and advertised is more like it) for leather interiors in convertible cars is RaggTop that I know of. It would be incorrect for me to say how well it works , as I have NO experience with it nor have used it.
Maybe someone will see this thread topic who has used RaggTop Leather Cleaner and Protectant Kit and comment on their experience with it and we Autopians, including you, can learn something.
Another is Mckee`s37 SiO2 Leather Guard, but again, I have no experience with it. Their website for that products states it has "UV-50 protection", but it, too, sounds like marketing hype, so you will have to make your own conclusions about its validity.
I will caution you about wearing sunscreen protectants on your skin. Make SURE you rub it in completely. Certain brand of sunscreen can stain clothes (cotton specifically) and if they can do that to clothes, they may do it to your leather. Granted leather is a type of skin, but it is not human skin.
I will also high-jack this thread, meaning I will talk about something else other than your thread topic question, and advise you to take care of your convertible top material as well.
Chances are, your top will be up more than it will be down, unless you live in sunny California, Arizona, or Florida. I assume this material is canvass/cloth and not vinyl on the MB E320. I have used RaggTop Fabric Top Kit with good results for that top material, but I think Aerospace 303 (now owned by Goldseal) makes a cleaner and protectant for that convertible material as well that is preferred by some Autopians in this forum. You may want to do a search within this forum and make your own conclusions for taking care of your top. Just follow the label instructions. One piece of car-care equipment you SHOULD have in your car cleaning arsenal is a brush for cleaning and shampooing that top material. Again, RaggTop makes small or large-sized round brush with soft, NYLEX bristles specifically for cleaning tops,
RaggTopp Premium Convertible Top Brush
but a quality brush will do, as long as the bristles are SOFTER, like horse hair, and not hard, like plastic bristles for scrubbing carpets. It can make all the difference in cleaning and keeping up the appearance of that nice MB`s top.
I also assume the rear window is real glass and not clear vinyl (it IS a Mercedes-Benz after all) because photos I have seen in replacement tops for that model say to reuse the existing window and the window has rear-defroster wire lines in it.