I had them all anyway, except the P-51D which I have just added via this sale for a paltry $9.99
http://www.digitalcombatsimulator.com/
DCS: A-10C Warthog – $19.99 USD
DCS: P-51D Mustang – $9.99 USD
DCS: Combined Arms – $20.99 USD
DCS: Black Shark 2 (Full) – $19.99 USD
DCS: Black Shark 2 (Update) – $9.99 USD
Of course, the "OS" for this sim is available for nought, along with the Su-25T Rook
http://www.digitalcombatsimulator.com/en/world/
btw, the P-51D currently has no WWII era props to fly with/against but some are promised for the future. It is very highly polished and detailed, though - and comes with training sorties very much like those that came with their excellent A-10C (i.e. with a virtual flight instructor and indications of which instruments or controls the instructor is referring to throughout each lesson).
Recommended!

PS - I would leave Combined Arms for now, unless you know what you are getting - it's best use is as a human JTAC for the AG pilots in multiplayer scenarios, though it can be used as a basic armour game.