There was another thread on this recently. Gregs210 posted a good checklist for common issues at Benzworld and BenzObsession, some you can check yourself, some you may want to leave for others. Here's a link: http://benzobsession.com/phpbb/viewtopic.php?f=31&t=3438

Plus two on a good PPI, it's money well spent. Things that are found will help you chew down the price. Plus the mechanic may find something really ugly.