I too have a 1976 450SLC. I picked it up last year in AZ and drove it back to MN. It has 21K original miles. Needless to say, it has spent most of its life in garages.
The car was purchased new by my wife's grand-uncle for his wife to use. However, she never obtained her drivers license, so the car sat in a garage for the first dozen years of its life. My father-in-law then bought it from him and he didn't drive it too much either (he thought he was getting a SL). I got the car from him last spring and I store the car during the winter.
I did have to spend over $1,000 to replace lots of fuel system components. Unfortunately, no one used any fuel stabilizer and the long periods of time that the car sat took its toll. I have also changed all fluids (and filters) and replaced them with synthetics. Oh, and the a/c needed a seal replaced and R-12 freon added. New Michelin MXV4s were installed a few years ago. And I needed a new battery before it could be started.
The 450SLC does allow the whole family to ride in it, as it has a back seat. It cost more than the SL when new, but Tim is right in that it is the Rodney Dangerfield of MBs; it gets no respect. But it really is a more practical car than the SL and built like a tank. The '76 model does get real hot under the hood. Not sure if it is worthwhile/possible to move the CATs downstream, where the '77s are located.
I am taking this car on a long driving trip next week and I'm sure I'll have more than my share of funny looks and stares. You can't say that with most other cars!