Below 7k its definately a W220. Im not sure I can help you but £16k may get you into a leggy early W221. Others will say that the V6 is great, I may not but read through the technical sections and get yourself clued up with turbo seals, inlet port shut off motors, engine bearings and 7g valve blocks. One thing to consider is that although the W221 is the newer model, you will be buying into an early version where bugs weren't ironed out. A late W220 will be as good as that model ever was, which was actually pretty good, and might be more peice of mind. The I6 and 5speed box are very highly acclaimed and whilst not as smooth as the V6 engine, are hardly rough. That being said, the W221 is a marvellous car and we dont hear of many issues with it.
Airmatic wise. Operate the buttons, the car should raise and lower. The car should not be sat on its belly, nor should it be sat one side higher than the other. Airmatic is a great system when it works. I didn't know it was a standard fitment on S320cdi, thought only the top 500 models plus got it.
