For Cliffs of Dover get the Team Fusion patch from the Air Tactical Assault Group (ATAG) site.
Rise of Flight should be okay with your key and downloading.
P-51 by DCS is great, it is beautiful, but it is a DCS product and so still isn't a 'jump in and fly' aircraft, but out of all of them it is the simplest.
Flaming Cliffs 3 I've not played yet, but is attractive and yet more simple to fly than the other DCS stuff.
Don't know if any of this helps you, but in order of preference for me I'd go RoF, FC3, P-51, Clod.
Edit: Oops, forgot War Thunder. It's more game than sim, and it is online MP play only.
__________________
|