Hi Colin, Orbx True Earth UK comes in three stages, EU South, Central and then we are awaiting the release for the North which will be Scotland. That is all you will need, no other Orbx Sceneries required. Default airfields are the ones already in X Plane 11 which are so so, like any other default airfield really but UK Sceneries are being ported to X plane 11 and as it is a different platform to FSX and P3d you would have to purchase them though would think that you may get a discount but not sure.
The Orbx Scenery of True Earth is 2018 era so it is very much up to date.
I have FSX still on my computer as I enjoy flying the old classic aircraft I have but..........FSX with Orbx England Wales and Scotland does not compare to Orbx True Earth so my plans are to go to P3d sometime next year.
I am presently really enjoying X Plane 11 and it will become much improved when Laminar release 11.30 (currently on 11.26) and one aspect of the upcoming update is you will be able to customise your joystick using response curves which will make handling the aircraft a lot better than it is now (no more skidding down the runway for one thing).
Back to your question of sticking with FSX, if you can keep FSX and run another flight sim then do that, it would be a shame to have to bin what you already own. I have spent hundreds of pounds on FSX but after flying with True Earth I find no enjoyment with FSX but still want to use the aircraft I have purchased so will buy P3d at some point next year and just write off the other sceneries.
One aspect of True Earth is that if you buy for X Plane and then decide to buy P3d (as I will) then you don't have to purchase True Earth again. You will be allowed to install it into another platform as well as the one you have. Hope that makes sense!
Regards
Nigel.
I used to be an optimist but with age I am now a grumpy old pessimist.