FSX is fine. I have FSX, and FSX-SE installed (see my signature specs), and I can't remember the last time I had a OOM problem with FSX/SE - in fact I've rarely had them in the first place. And I still like using FSX/SE.

AFAIK, most CS FSX aircraft are not compatible with the 64bit Prepar3D v4/v5, so you'll need to look at buying the aircraft that are made for them (757 Captain III, 1011 Captain II, 767 Captain II. and 737 Captain. The CS C-130 is now a default aircraft in P3Dv4/v5, so that one you get when you buy the sim.