Chris,
93-95 had the engine wiring harness issue.
90-92 are good cars but please make sure that on any R129 that you check out the soft top hydraulics by raising and lowering the top multiple times and asking if any of the hydraulic cylinder seals have been repaired.
IMHO, I would go for a low mileage 99-02 Sport model with a SL2 option package. Prices are less than $30K for a $90K car with a 5 speed trans and you shouldn't have ANY concerns. Here are a couple of beauties
http://www.benzworld.org/forums/2571835-post7.html that I have no interest in. Just passing the word.
But if $30K is more than you had in mind I would like to suggest a 86-89 R107 classic. But, check the center A/C vents to make sure they work. I have never seen a Mother that didn't love the R107. Please see
http://cgi.ebay.com/ebaymotors/1989-MERCEDES-BENZ-560SL-NEW-TOP-NEW-A-C_W0QQitemZ280159180005QQihZ018QQcategoryZ6338QQssPageNameZWDVWQQrdZ1QQcmdZViewItem that I have no interest in. Just passing the word.