The best years for the W140 are 97 - 99. Those years are the last 3 years of the model run and therefore are the most sorted out. The S320 is not an all aluminum V-6. It is the venerable M104, Inline 6, variable cam timing, 228 HP. In the opinion of many, one of the best engines MB has ever produced. I had a '93 E320 with that engine that had 254K on it. Engine ran perfectly. My '92 300E M103 has essentially the same engine, with a different head and valve train, 242K and running strong. The M104 engine in the S320 is very easy to work on due to the size of the engine compartment.

The car has the 722.6 Electronic Transmission that is allegedly "Sealed for Life". A '99 will have software upgrades and the latest build specs. This is a good thing. MB now recommends changing tranny fluid and filter every 40,000 miles or so.

What are the issues? Potential A/C Evaporator leak, cost $3,000 as the entire dash must be removed. Head gasket leak, probable replacement required , if not already done. Lower control arm bushings will be due for replacement, say $500.00 installed. That's about it.

I swore I was going to stick with 124's, but then begain to research the S320's. I found one locally with 106K, fully dealer serviced with all records. It is light years more comfortable than the 124, great seats, great stereo, CD in the trunk. Wonderful road car. Gets low 20's on the highway at 75 mph. Has laminated glass on the side windows. Very quiet on the road.

Hopefully, your target car has records. Spend the money for a Pre-Purchase inspection.
I paid $11,800 for mine 8 months ago.

Steve
'97 S320 116K
'92 300E 242K