It's an oil and filter change, the rest is checks. Wheels don't even come off for an A service.
Cost at a dealer £200-£250, though it gets cheaper as the car gets older.
In addition to A and B services, other items need replacing from time to time and this is known as additional items.
These include air filter, fuel filter, cabin filter, ATF, spark plugs (for petrol cars), brake fluid, etc.
When such an item is due - depending on age and/or mileage - replacing it will be an additional cost on top of what you have been quoted for the service.
And given that your car will be just over two years old, brake fluid change will be needed.
Other items from the list above may also be needed depending on mileage, and brake pads and discs may be needed depending on wear.
You can check what exactly is needed for your car's age/mileage online on the MB site.