450SL: Avoid '75 and '76 models with cats attached to the exhaust manifolds in the engine compartment* and '78-'80 with the doomed to fail Automatic Climate Control (1)

380SL: Avoid '81-'83 model with single row timing chain. *

Best R107 model? IMO 560SL Many improvements over prior years. The list is here: Mercedes Benz 107 SL SLC Roadster Information Specs Page

* North American models only

(1) All North American models and some rest of the world.