I am pretty sure that the E420 / 400E never came with a turbo (at least not from the factory). You are right, they are a lot faster at higher speeds then at lower speeds. I own a 1997 E420 (w210) and have not really had much trouble with it.
With regards to the 540i, you need to be careful and make sure that if you decide to buy the car must have the alusil block. The 540i's came out of the factory with what is called a nikasil block. This block contained a certain mixture of nickel in the alloy that resulted in abnormal wear of the block caused by the high sulphur content in US gas. As a result these cars were given a 100,000 mile warranty from the dealer and many had been replaced. If you are looking a car that has not had the new block put in (93-95 models) then you need to be careful as it is very expensive to do this on your own nickel (pardon the expression). I close friend owns a 6 speed 540i that was recently stolen and recovered. He is hoping that the car will be totalled due to the 2000 mile run that the thief took it on (damaged a lot of small items resulting in a lot of money), if it does get totalled, I would be very interested in the salvage title :-).

__________________
With best regards

Al