Having purchased a used Benz myself ('95 E320) there are a few things I would recommend for your piece of mind. First, have it checked by a qualified MB mechanic. Second, ask to see all maintenance records. Condsider doing a check with Carfax (you'll need the VIN for this). As for cost, you can check at kbb.com, carpoint.com or several other Web sites to get a good idea of the prices for which these things go.
Hope this helps. Good luck,