i looked at 600s before i bought my sl500. i thought, for a few grand more you get the extra power, extra leather, and all the options...but every mechanic i talked to, dealer or private, told me to stay away...even with a warranty. leaks from gaskets are often not covered by your warranty and hoses, etc. are definitely not (remember, this isn't a manufacturer's bumper-to-bumper). access is tight, and thus fixing those things not covered by your warranty will be pricey. he told me a small oil leak could easily run $2200 to fix. just call your local mb dealer and talk to a serviceperson. i've always found them very helpful and willing to give their .02 to someone needing advice.
also the 600s don't hold their value nearly as well - consider the premium the were over a 500 when new and how close in price you find them now.
'96 SL500 46k