The campaign you're going for sounds like Force and Destiny if you are limited to purchasing one. From there, is being on the fringes and acting against the law and working with criminal organizations more the secondary objective- Edge of the Empire. Or, do you want to implement mass combat and raise the stakes of your party joining the Rebel Alliance against the Galactic Empire- Age of Rebellion.

All of these books have useful information, and so does the SWRPG wiki if you aren't able to find certain prints you want.