In general, and if you want an AMG, the S63 (8 cylinders) is a wiser choice than the S65 (V12) due to the impending maintenance nightmares you mentioned.
About a year ago I saw a 2004 S55 (50,000 miles) going for $39,000 at my dealer so that's pretty much the highest you'll see. They have plenty of power.The S63's are a little beyond my budget. Maybe I need to look at the S55's?