Crap, someone said my name.
Coming from strike fighters, I’d say FC3 is a good portal. The avionics and physics are a bit more in depth than what you get in SFX, be this as miniscule as actually having to slew the sensors, to picking which radar mode, PRF, and scan volume are best for a situation, to major things like dealing with the doppler notch, missile guidance and physics, etc. Additionally the Su-25s, the F-15C, the A-10s, and the Su-27 all have what is called “PFM” (Professional Flight Model), which as good as any full blown DCS module (I can vouch for the F-15s).
I personally fly the F-15C, the F-5E, the F-86, and the Viggen. I’m more or less fluent in just about everything.
None. There was such a hulabaloo when FC3 was released (on the heals of BS2) that ED promised they would not charge money for another FC3-level simulation, and ED isn’t going to release modules they can’t charge for. Likewise none of the 3rd party developers have announced any intentions of making them. Everything from here out, for the foreseeable future, will be study sim level.
@Sryan put together an absolutely masterful guide on how to fight air to air in the F-15C. While the examples are more or less for this jet, once you can grasp the concepts beneath them they apply to just about every Gen 4 fighter in one way or another.
Qualifying things helps. updates happen more frequently and modules release more frequently. I won’t get into sales.
Where applicable. The Russian systems tend to remain the same because the Soviets built their avionics for maximum commonality. An F-15C’s avionics are nothing like an Su-27s avionics. The Su-27 is however very similar to the MiG-29. The A-10 is nothing like the Su-25, etc.
So I think FC3 would be a good start. Another question to ask yourself is what do you want to do in DCS? Right now one of the things about the sim (it could be argued, a weakness) is that there is a massive variety of aircraft that are unique and do different things in different ways.
Do you want to do air to air? air to ground?. What type of those do you want to do? There are miles of difference between how a Mirage, an F-5 and an F-86 fight. Likewise the way an F-86, a Viggen, and a A-10 drop bombs is all wildly divergent. None is less valid than the other (well, except the A-10, far too slow).
Finally if you ever want to try any of this, drop myself or @Fridge a PM. With a few days notice we can throw together a scenario that can demonstrate the merits of a module.