My full brain dump.

I believe that the 3-valve/cylinder engine introduced in 1999 had less HP (302) than the 1996-98 4-valve (315).

Here's an article by Ken Rockwell that discusses the changes from year-to-year. He likes the '98 model.

http://www.kenrockwell.com/sl/r129history.htm

I like my '96, but the PO had to put a rebuilt transmission in it at 90K miles.

My S500 coupe (same engine and trans) had the problem with the electrical connector seal leaking allowing ATF to wick up the wiring and fill the controller. It would then randomly lock up the torque converter at a stop and kill the engine. I am not making this up. At about 110K it started to slip when moving off from a stop when cold.

I replaced the oil tubes in the coupe at 100K - one had blown.

1996 was the first year for OBD2 diagnostics. People with Star Diagnostics have told me that they got steadily better over the years and were "pretty good" by 2000.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe