 I always wanted a S600 Bi turbo but the 130K price it wasn't in the cards. So I oped for a 10 + year old model (2003) with about 90k miles for about $10,000. I have owned a E420 and C280 both very good vehicles and lasted well into the 250K mileage range.   
Unfortunately the S600 is not well built at all.  I will start with the interior: Drives side door window regulator broke lose from the door itself, cheap rivets holding it on. RR passenger door lock not working.  Hood & trunk struts wore out. Rear trunk assist pump failed. 
Exterior/mechanical issue;  ABC system not working, bad accumulators and valve block, coil pack right side failed, valve covers leak, transmission conductor bad. 