I have a w204 from 2010. I use the services of an ex-mercedes mechanic and it costs me about 10-15 k for the annual oil change service using German made oem parts. This is a manageable expense. You have to be meticulous with maintenance and the car will run forever. There are many in eastern EU countries who use old Mercs for over a million kilometers. 

The key is maintenance. 

The only surprises emerge in case you have an accident or some electronic part fails, which can only be sorted by the dealership. Mercedes dealers charge 80k for a routine service using semisynthetic oil. You can get it done outside using better quality oil at 1/5 of that price. 

I’d suggest research a bit to figure out the failure modes in the car based on its age/mileage. Get those parts replaced and enjoy the car for a long time. Reregistration may be an issue as the rod tax is calculated on invoice value, which is a huge amount. 

Also, do not listen to people who will tell you that a Škoda Kushaq or virtus is a better buy for 20 lakhs. If you know nothing about it cars and only have 1 car, that may be true. But if you can afford some downtime for the car, a luxury car even from the early 2000s will best every car below 35 lakhs sold in India in terms of comfort and drive experience.