Different geographic regions will bring different prices, especially where a car has 4Matic. They tend to bring more in snow country than in, for example, Florida. Your best bet to determine a fair price is probably KBB, Edmunds, or Truecar, which can price them in your area. A KBB dealer price on a 45K mile '11 S550 equipped like yours in the 27601 zip code, and in "good" condition, could go from $33,900 to $39,700, with a suggested retail of $37,873 and a fair purchase price of $36,800. I would look to see why this car is priced below suggested retail, and consider that condition, dealer inventory, end-of-month sales quotas, length of time on the lot, and the prospect of having to pay taxes on inventory can affect prices.
Jaybm is absolutely correct on PPI, and on the transmission flush at 40K (unless it has been revised. My '10 was due a flush at 40K, and my'13 at 60K - but the '13 has been revised now to 80K in the latest bulletin). Check to see that it has been done, as well as the 5-year brake fluid flush. "B" service plus those two items ran me about $800. If not done, use it as a negotiating tool.
Maintenance records will be a factor that KBB, etc. won't show; an MB dealer doing the PPI should be able to get them for you, especially if you plan to do business with them in the future. Also, get a CarFax report, look for accidents (and significant maintenance problems). By comparing the two on my '13, I was able to tell that a first owner had probably taken a curb early on - a wheel and a tire replaced at 600 miles, and several subsequent early tire replacements and many alignments and much wheel balancing. Those disappeared for the second owner, but she complained endlessly about the brakes (which dealers said were good) - I came to the conclusion that she was probably unfamiliar with the way lane keeping corrections worked in the driving assist package. Under the right conditions when you start to leave a lane, the steering wheel vibrates and the car brakes to pull you back into the lane. This may have happened to her a lot, because the front pads were replaced at 30,000 miles; since I bought it, brakes have been fine. However, when I test drove the car, it had a slight shimmy (possibly the reason for the first owner's repeated balancing and alignments) - that was traced to a slightly bent inner lip on one front wheel (possibly even dating from the curb incident several years before) - when it was fixed, the shimmy was gone. But the CarFax and VMI reports I got helped to suggest the problem and the solution.
Without Distronic Plus you also cannot get options such as blind side assist and lane keeping assist (features I truly appreciate in my '13).
If you have the Multimedia Interface connector in the glove compartment - the one that allows a 30 pin iPod connector to be plugged in - you can connect an iPhone that has Pandora, and access it that way. If your iPhone has a Lightning connector, you can use an adapter with the 30-pin connector. Because the selections on the phone must be made manually, it is not as nice as a full integration, but at least you can access it.
FWIW, I bought my '13 6 months ago - 35,000 miles, AMG+1 sports appearance package (+1 = 20" wheels vs 19"), Distronic Plus, the two driving assistance features (no night vision), drive dynamic seats, rear side window sunscreens, and RWD - for about $44K. I love it. I also loved my nearly identical '10, but traded it to get the Distronic Plus and driving assistance features that the '13 had and the '10 lacked.
Airmatic has improved over the years, as far as I can tell. We have relatively few complaints in the W221. The system is similar to the W220, but has continued to improve over the years - but any suspension system, even coil springs, must be maintained. Arnott makes great replacements at less cost if you need, and they do make them for the 4Matic as well as RWD.