The mileage you do is irrelevant (but it's similar to mine, as it happens) as when the summer tyres are in use, the winter ones are not. The initial investment you make of the extra set, should be partly offset when you sell the car, as you can sell the extra wheels too, or sell them independently. The difference in winter grip, and the fact that aquaplaning becomes a thing of the past, is what matters. I have lost count of the number of times that I have had to brake hard or swerve in cold/icy/snowy conditions, that I guess I would not have made it out in one piece had I been running summer tyres (which I used to run all the time). Admittedly I live in Ruralshire where the roads are narrow and rough, but rain and ice happens in towns too. Running winter tyres is all about safety, and if you can afford to buy a Merc, you should be able to afford to increase your chances of driving safely in the winter. Just my two-pennies worth!
Anyway, you asked about all-season tyres, and not only are they more expensive (against comparative-quality summer tyres), but they wear at a much higher rate in the summer, so either way you'll be paying more. Also, you'll be running the same width and profile of tyre in both seasons, so there's another compromise that you wouldn't have to make if you ran two sets of wheels.
I'm not sure about the conditions in your country, but if they are anything like typical UK conditions, if you only want to run one set of wheels/tyres all year, you'd be better off safety-wise running a good set of winter tyres all-year, and taking a hit on the extra wear in the summer. This is my blog-post on the subject, which explores the cost of running two sets of wheels >
http://www.sinclairdesign.com/b/blog/piste/winter-wheels-and-tyres/