Oh and some of the things I noticed while looking for my R320 a couple years ago, most of these modern Benz's were leased and not well treated. Lots of door dings, salt stains in the carpets, cola and coffee spills (the newer interiors that are not black DO NOT come clean, stains are stains!). Look out for repaints, most have damage (way over half of the R's I looked were repaints to some extent, I looked primarily at Benz dealers so much for star certified and being damage free!). Don't trust the star certified program, have it inspected by someone how knows the vehicle and seek a detailers advice about the bodywork and paint. You should never pay market value for something that was repainted, and few bodyshops can paint anymore or repair to a price.
Look for power steering rack leaks out the input seal (steering column), oil leaks from the topside of the engine usually looks like rear main seal however it is dripping down from above through drain channels in the block or intake valley-crankcase breather vent hose and related seals and the oil cooler are usual suspects, the oil cooler is a huge repair 12 hours labour. Check the tailgate power operation common issue. Drive it pay close attention to the transmission, under light acceleration they sometimes misbehave and do funny shifts 1-2-3-4, usually higher gears are ok. Lots of service bulletins on transmission issues, expensive dealer only repairs as everything is internal including the computer. Valve body replacements or reteaches are common fixes. Air suspension is going to cost, make sure you budget for air bags at some point if so equipped, they must be fixed promptly or replace the pump from overworking too. Feel the tires, run your hand over the tread they tend to chop really fast and make noise, I have not found a cure for this yet these vehicles have huge camber and it is hard on tires. Brakes I find way above average as the engine has great compression/lots of engine braking and seldom need them. Look at the passenger side under the hood just below the windshield, the heater air intake has drains down there and sometimes they are blocked causing lots of electrical issues/interior floods. Lift the carpets and check out the sub floors, there are 8" deep compartments below the front and rear seats holding the battery and all control modules-a flood victim would pose a serious problem or salt contamination/some dumb detailer that used a hose to wash the carpets (yes it happens!).
Choose your options carefully, for me it was essential to have HID/Bi Xenon headlights, they were not that popular-a pricey option. Wait for the right one to come along, it took me 8 months of looking to find a good vehicle, I looked at every one that came up for sale, usually at Benz dealers no matter the options or colour etc, and bought the first one that was clean and genuine (and has every option as it turns out).
I manage to get better economy with the R then the older diesels especially in the winter, not a lot more but at higher speed with all wheel drive, and they start great in the cold with lots of quick heat.
But be warned, when somethings fails on these units your not going to fix it on the roadside, nor tow it home yourself without a flat bed. They are also extremely sensitive to fuel quality issues and water contamination, and forget about biodiesel or WVO. The post injection for the diesel particulate filter dilutes the engine oil and it would gum up rings and sludge the oil. There are no issues known yet about this emmissions system, but replacement costs are very high at this point.
I have the time and experience to work on mine, I enjoy babying it and going above and beyond the maintenace requirements, I read up about issues and try to be proactive as a newer Mercedes is something else to drive/comfort/power. But that being said, the old 240D is more fun to drive (way more road feel) and special, the new vehicles are very isolated! It is not something easy to work on either, the V6 is very cramped, servicing costs are high even for DIY (10 quarts of oil, filters, special tools etc). If and when something goes wrong I have the time to research and take time to fix it, if I was working full time that would not be the case and would likely not own a modern diesel. The parts for it are not easy to come by, very few parts are available online, dealers don't stock much and not a ton of info online either for engine problems etc.
I have two years and 33k miles of experience with our R320 (has 100k now), and decades of diesel experience with old Mercedes/VW etc.