The 500 sel/600 sel were designations used in 92 and 93. I t was changed to S500 and S600 in 94. The 600 has the same problems and everyting is crowded in and more expensive. The 600 engine is two 300 engines sharing a common crank if you want to draw an analigy. I think Steves quote for the AC is conservative, the dealers are worse, and even at my dealer the people who do the AC are a selected few.
They are wonderfull cars I still have one (12years) and have spent a lot on it, the later ones are better!