If it really is well maintained it sounds like a steal at the price. I have seen excellent examples go for twice as much. These are great cars, if it is in as good a shape as you say you won't be dissapointed. If you want to be sure book a couple of hours at the dealer and ask them to go over it head to toe to find anything they would want to fix now, or things they think look like they might only last a short while longer. Its no guarantee that's for sure, but it should give you a good idea. Have you taken it on a good long test drive? I would suggest a full half a day in the car at least, a couple of hours on the freeway and a few around town to really get the feel of it.
1995 E420 - retired
1986 420SEL - retired