You don't really need the whole Flaming Cliffs 3 package, it works with individual FC3 modules too but you need to buy the module with exactly the avionics you want your mod to have. By example if you want your mod to be a pure air to air western platform and that's it you can buy the single F-15 module instead of paying 60$ for a bundle of jets you won't use.
But if you want to mod a lot of platforms with different roles for both sides then yes you want the whole FC3 or you give it up because it way to simplistic and code your mod DCS level (somewhat).
FC3 moding it's a great way to add AI aircraft for coop missions and so on .. i remember we had x15, sr71 and other exotic planes to escort or shut down.. kind of fun.