Yep Comand is optional (well, in Europe at least). It is only TV/sat-nav so if you don't need it... It is often on the S500 and above however.
The first Keyless-go (first yeat I think) had some little troubles sometimes, and did not recognize the card from some angles of the car very well. It was corrected quickly, so in 2000/2001 there are no more probs.
Notice also that keyless-go + darkened windows = harder for the system to recognize the card, it needs more time and you need to be really near the car.
Other than that I don't see any problem on the W220.
I would say the 320 is too small, the 430/500 is much more adapted. The 320 cdi is smooth and refined, a bit short on power, but I don't like the 400cdi at all: noisy and not very refined, and some little turbo troubles.
The finition is very good, but some details are a bit...cost-cutted, like the door panels for instance.
The comfort is just divine, it is a pleasure to drive such a car. You can travel the world without beeing tired at all... It is not sporty, but it does not feel heavy, it is really a masterpeice.
Like mpelchen said, it is important to choose the right car. Go and see them, testdrive them, to be sure the car is ok. With a warranty, one year is perfect but 6 months should be enough, you will see id the car is good or no. If you have no probs in 6 months, it means that the car was well-serviced and you won't have troubles with it normally.
I think if you take a car with a clear past, not too much kms, a service-book correctly filled, and a warranty, you will be travelling in one of the best cars ever and enjoy the car immensely.
