If you really want to go double action, I recommend the Badger 100G (or LG) with the medium tip.
With internal mix, you'll generally get a finer spray than external mix.
Gravity fed lets you use less pressure for spraying plus I find them easier to clean.
I recommend a medium size needle due to the thickness of modeling paint pigments.
I really don't see the need of multiple needles if you're only doing modeling.
I have two airbrushes, a Paasche H and a Badger 100G. I used my Paasche H the most.
-Jesse