XBS,
You may be hard pressed to get a S500 in the low $50K range.
I purchased a 2000 S430 about six months back (I moved up from E class) and I have been very pleased with overall performance to date. Savings was about $5000 over the S500, which is identical to the S430; S500 has 27 more horsepower and 44 ft-lbs of torque. Your call.
You're in southern Cal, but if you venture into the mountains or where it's wet or slippery, the 4Matic on-demand system is something to consider. This became an option on the 2003, and I could certainly use it here in Canada in winter.
The airmatic suspension system is superb but very costly to repair if it fails after the warranty period. You may want to consider an extended warranty if you are planning to keep the car beyond the warranty.
6 CD player is a must.
HID if you like the look.
Integrated phone and voice command if you're doing business while driving.
"Soft-closing" doors is a nice to have. Rather than slamming the door, you close the door to the latch and an electric/pneumatic mechanism latches silently.
Getting a 'new' car is always an exciting time - good luck.
