I went through this debate recently, and bought a 400e instead. Why? Nicer performance, and a few more features ad a slightly higher cost. That aside, neither the 300e or the e320 is especially quirky. In any event, go for the least miles you can find and have the vehicle inspected by a good independent shop or a dealer. Also, consider a warranty. If you go through a dealer, get a Starmark warranty. Otherwise there are several after market warranty companies around. With the Starmark warranty, the car must be a ’93 or newer.
Good luck! You can’t go terribly wrong with a MB!