If I were you, I would pass on the S320. Test drove a 98 S320 for a week before deciding to get the 99 S420. The S320 is way underpowered, acceleration to 40 went all the way to 4000 rpm. Also the fact that it has stainless steel headers and at 4000rpm they release their slight ringing noise which can get a bit annoying. The v8 is great, it has gobs of power and is quiet. According to my mechanic, the best W140 is the 98-99 models. Up till 97, these cars had evaporator problems. If you really want the car, check these areas:
head gasket $1-1.5k
wiring harness $2k
Lot of these old W140s are being "dumped" because of the mounting costs of repairs.