I am not sure what you meant by ' double evap'...... I would sure not change out my Evap unless it was leaking.... remember that the place where change makes the most difference is Flow across the Condensor. If you have great flow over the condensor ( with it normally clean and with straight fins ) then other parts of the system can be a little below par and you still be nice and cool. This is why I keep emphasizing the larger fan . I do think that aftermarket fans are available which are have greater flow than the stock MB fan.
R134 is automatically ' below par' compared to the R12 just due to the makeup of the molecule with regards to cooling efficiency... so I sure think that the extra strong cooling fan is necessary if one changes to R134. I do agree with Larry about people keeping R12 if they are able , since this is what the system was designed for.
There is nothing wrong with changing the hoses... I was just mentioning it due to the cost. New hoses of the right kind are the cat's meow.