Unfortunately the new MB cars are pretty much along the same line of quality as their new buddies Chrysler and are approaching "disposable" status. Hopefully they'll get a turnaround and return to their reliable, value-holding roots but it's unlikely in the state of acceptable business practices.

But back to your question: A typical top-end rebuild is both common and straightforward. Since you mention your engine is sleeved, that feature adds longevity to the block and makes such maintenance a simple affair. The reason your pistons aren't being replaced is that the only thing that usually wears on a piston is the rings, and those will be replaced. My guess is that the sleeves may be showing some scoring as a result of debris from whatever may be happeing in the head, or as a "good service" measure by the shop. Again, not uncommon. The replacement of the head (the 230 should only have one) could be due to any number of reasons, such as warping or a known design flaw. They'll probably use a "complete" head which will already have the valves, guides, springs etc. already installed. Unless you're on the cheap you don't want to re-use that stuff anyway in regular vehicles.

So from what you've shared it appears they're on the up-n-up. Be sure to get all the warranty paperwork and verify the guarantees and such on the work, just in case.