Have you called Auto Radiator Outlet at 1-800-252-0333? Or checked
http://www.radiator.com (1-888-radiator) I priced a radiator for you at Radiator.com, and for an '81 (they didn't go back to '80) their price was $216.00. They guarantee the unit for life, and also guarantee to have the lowest prices, and next day shipping. None of these places will do the R&R, but they seem to have very good prices for a new replacement. Oh, and there's also Discount Radiators at:
http://discountradiators.com/ or 1-800-443-1322, but I couldn't get their search engine to work for me.
I am researching the same thing for my '82 W126 300SD. I went to an independent MB shop, and after seeing their estimate, asked them if they did the work. I was told that they take it out and put it back, but for the actual radiator work they send it to a radiator shop....
The only thing that concerns me is that you say that YOU determined that you need a new radiator. Have you taken it to a shop to have it checked? Are you sure that your problem is actually the radiator? What kind of coolant do you use? When did you last have it changed? Maybe all you need is a thorough flush and a coolant change? You can always check out
http://www.radiatorinfo.com/ for more information.
And, when you do change the radiator and/or coolant, use the Mercedes coolant, or at least an "extended life" coolant that is Non-Green in color.