My advice has always been that if you are not capable or willing to do your own repairs, then no old car is really an economical option. Through the years most owners have neglected so much that you have a lot of repairs and maintainence before a used-car purchase is in "reliable" condition. The other issue is that you apparently are looking for a car that needs no frequent service and you can just drive and depend on it when you need it - that sounds like Toyota territory! My opinion is that Mercedes are pedigrees and need/should be treated as such. Such care in an old car is costly if you are paying someone else to do the work.
I sunk about $2,000-$2,500 in my car in the past 1.5 years to get it in really good shape. How the finances work out over the next few year I don't know, but as it stands I think all I'll need now is oil and fuel for quite a while.