I’m also a new owner of an R class but have had a 20 year history with mb vehicles.
I did a lot of research, and looked online at probably 80 or so R class vehicles before settling on my r500. I also had 2 taken to dealers for a PPI. I declined on both of those.
I strongly suggest you or anyone shopping for a used car to get a) the lowest mileage example you can find, and/or b) buy from a MB dealer who will offer at least a short term bumper to bumper warranty on the car.
In any event, look at the vehicle master report which shows all of the service details the car has had at MB dealerships. This is way better than a carfax report. Of course a carfax report is also good to show if the vehicle has been in any reported accidents and to verify the number of owners. A 1 or 2 non smoker owner is ideal.
Ideally you want to buy from someone who took their R to the dealership regularly and who was obviously obsessive about the care and feeding of the car. The research takes some time but it is better to buy form someone who went to the dealer regularly and thereby had ongoing dealer level service. As example, the one I bought was taken to a dealership nearly every week for washing and detailing about 4x per year. The previous owner had the oil changed about every 5000 miles, and regular replacement of all the other wearable parts.
I’ve put almost 3k miles on my R500; about 2/3 of that was driving it from the south end of the USA where i bought it, to the north end where my home is. The trip home was a lot of fun. The car is a fabulous, extremely roomy, and a fast hiway platform. It has more cargo space than even a GL class, which is why I bought it.
But with just about any older MB you can expect to spend around a couple of thousand per year on maintenance. That is just the way they are. But if you are willing to keep up with maintenance they are very reliable and last and last and last.