2001 e55 w/ less than 30,000 miles 1-owner
 

Under $20,000? Other than spring perch failure, why is this a bad idea?

It's not. Those engines were well-built, the w210 body has a few issues, but most of them are known and many of the issues were fixed by the 2001 body style. The E55 is a hell of a car, but I'd expect further depreciation. The engine bits cost more, the brakes cost more, the suspensions stuff costs more, and you don't want a drag-car. Otherwise, the E55 is quite a bargain for what you get.

I actually saw a W210 E55 today when going to the auto wash. :)

The N/A cars are pretty quick for what you get and are not common at all. As I recall these were the last of the fully hand built AMGs. I can't confirm when I was told that, but eh. I'd take one for the right price.