Nuts! I was typing you a reply and my cat stepped on the keyboard and erased it.
Anyhow, a 123 is the chassis number given to all MB cars that look like yours from 1977 to 1985. There is a cool sight that has a link that explains the numbering system. I think it's http://www.benzowners.org.
If you are interested in learning more about your car go to Pep Boys and get a Haynes Manual. Between this forum and that manual I have learned alot and saved a fortune.
As for as $7000.00 for a '79 280CE, I'm on my way to where you live to sell both my cars. Kelly Blue Book only goes back to '81. But even an '81 280 SE in excellent condition retails for less than $4000.00.
It is possible a collector might pay big bucks for a pristine one. However, yours now has a motor whos' numbers don't match the car's. To someone willing to pay big bucks, that's a big no no.
But don't worry. You have a cool car you like, at aprice you are comfortable to pay, that will last longer (given proper maintainance) than your neighor's new Ford or Chevy.
You might spend $1000-$1500.00 a year in repairs, but that's alot cheaper driving than $300-$400 a month in car payments! Best of all, when your neighbor decides to trade that new domestic in in three years, he'll owe more than it's worth. You on the other hand will be able to sell yours for pretty much what you have in it.
Aint that cool!(and smart)