Did the overheating occur all at once or was it a gradual trend?

Just by the info you've provided, it appears that the coolant is not circulating which would lead me to believe that the water pump is not functioning properly. You did say you replaced the thermostat (assuming it wasn't installed up-side-down) and the radiator which rules out those two components.

Did you pour in a 50/50 mixture of coolant to water?
Did you remove all the air from the system by bleeding the cooling system? There is a bolt just behind the thermostate housing that sits at in about a 30 degree canted position.

The worst case scenario is that the head gasket is leaking and allowing air into the cooling system which would cause overheating.

Herb
'82 240D
114,000 miles