You can't practically convert a non-turbo to a turbo without replacing the engine. Personally, I like the 82-84 300D turbo (the 85 is a little different, a little more complex, and has some unique parts). It's all about condition and maintenance records. If you are not very familiar with these cars, have a good benz shop do a pre-purchase inspection.

The 240D (auto or manual) is a cool "around town" car, but don't buy one for extensive highway use.