I highly recommend the 400E. I've had mine fore 6 years. Yes, maintenance is more expensive, mostly because it's a bit harder to get at things under the hood. On the other hand, the M119 engine is, by all accounts, bulletproof. It's the kind of engine technicians describe wistfully (or at least mine does), they like it so much.
A couple areas to focus on. Check the operation of the cruise control for an extended drive, if the car has traction control. The throttle actuator is a known problem area and a $900 USD fix and will show up as the car going into "limp home" mode while using the cruise control.
Also, the AC evaporator may leak, being made of aluminum, and this is a $2000 USD fix.
Yes, routine mainenance can be expensive AT THE DEALERSHIP. You would definitely want to find a good independent to work on it.
ALso, get a good pre-purchase inspection so you know what you're getting into.