You may have a hard time finding a 400E under 17K that has not been hit, frame damgaged, etc. There was no "re-design" of the engine in the 400E's. The engine management was a bit different on the 92's and early 93's such that those cars were faster on WOT than their later brethren. You do get lots of power with the 400E, but I almost guarentee that you will be wanting to put more rubber on whatever you buy. The 300E's handle better and feel almost like a 4-door go-kart whereas the 400E's feel heavier and less communicative. 400E's have better brakes, more standard luxury, and about the same gas mileage as 300E's. If it is your first benz, go with the 300E.
One thing I can recommend before I go into ad nauseam detail about the two...read the MercedesShop reviews on both of these cars. Click on "featured cars" in the box to the left on your screen. You should see the reviews there. Review those and if you have any more questions please feel free to email or post to me.
Hope this helps...Lee