88' 560SEC Evaporator OEM or Aftermarket, Copper or Aluminum?

A/C on my 88' 560SEC works great...when it has freon. Mechanic said the leak is from the evaporator and recommended OEM.

What is the original evaporator made out of (aluminum, copper or any combination)?

Which material is better?

Should I go OEM or Aftermarket?

It gets very hot down here so I don't want to change from using freon. Also cost is not a factor, quality and durability are. But I have read that some original MB evaporators corrode from the outside in!? To change the evaporator is a difficult procedure so I want the mechanic to do it right the first time.

Thanks for your help.