If I understand correctly you just want to cool the attic... if so the ductless system is the way to go.
But seeing as this thread is several years old you probably have already resolved the problem.
Many factors in switching a old heat only system to A/C... long story short cold air needs larger ducts.
Also if I had the luxury of starting from scratch I would go with a package HAVC only because you don't have to worry about leaks at the connections.