Thermostat? You know, I just have to ask
Could also be a water pump problem. If you paid more than $150 for the radiator you paid too much... I can do an M103 radiator in about 10 minutes blindfolded... Well, maybe not that quick, but close...
Also, your car should not have green coolant if I'm not mistaken it should be clear or transparent... Zerex G05 is what you need bro...
One more thing, the dealer doesn't care about how much money you spend. If your going to have an old car like this 300E, it's time to start learning how to wrench. All the items you just replaced @ the dealer are inexpensive (other than the radiator).
This is a great place to start learning. I learn something new almost everyday around here...
-GH