With Mercedes, most people would say the newer, the crappier. The golden age of Mercedes for many people started in the 1970s and ended in the mid '90s. Each model of Mercedes is going to have its own common issues, pros, and cons.
If you are looking for a reliable car to take yourself reliably and relatively economically to work in back, with a sporty feel, your best bet in that price range would be a W124 or W126 with the M103-M104 inline 6 engine. If you don't mind a dip in fuel economy and arguably higher maintenance, a V8 version of the two. If you want extra economy, diesels are another option.
Which car floats your boat more? I gave links to two ads of the cars, one of which I'm selling. Don't hold it against me.
1991 300SEL
http://www.peachparts.com/shopforum/showthread.php?t=269097
or 1995 E320
http://cgi.ebay.com/ebaymotors/1995-Mercedes-Benz-E320_W0QQitemZ360214755355QQcmdZViewItemQQptZUS_Cars_Trucks?hash=item53de78f81b
