I can only go by a well know UK airport developer who stated the SDK is still work in progress not complete at the moment, and totally different why of doing it in MSFS so no your FSX P3D scenery will not work in MSFS, that will not stop some trying from past experience.
And the price of MSFS includes the + in app purchases in just like steam DLC i suspect, and the boxed version will not ship till after the 18th release date.
IF your not sure and want to try before you fly pickup a Game pass for a month £1 give it a test on your PC then by outright if you like it.