I personally thing that 18.5lacks for a 1996 classic spec manual E230 is a decent offer. I wouldn't advise you to wait for it to cross the 20lack mark because it is too old and under speced to do that now.
A dealer would only offer you 16 for it. your car has reaced the point where either you hang on to it and let it slowly depreciate or sell it just under what you wanted for it and cut your losses.
Whatever you decide, good luck, it's a great car.