The 400E has the upside of V8 power, upgraded brakes, and most of the E320 options as standard. You will probably want to upgrade the tires (or wheels AND tires after not very long at all in the 400E. The physics of 3700lbs of car packing 300lb-ft of torque and 275HP rolling on 195/65/15 touring tires makes more rubber a popular mod. The usually griped 2.24 differential keeps mpg pretty close to the 6-cyl. Expect about 18-20mpg in town, 24-26mpg on the highway. Note: the V8's will run hotter than the 6-cyl cars. Search the shop forum for references to the 400E, E320, etc on past discussions.
Hope this helps...Lee