Hi James
Great to hear from you again.
My personal set up uses P3D, along with Active Sky Next (ASN), for real world weather and the indispensable FSUIPC.
Lockheed Martin have made significant changes to P3D, so that a lot of the processing has been transferred from the processor to the graphics card (in the same way that many PC games work today). On my ageing system (Core I7 920 ((2.67GHz, OC to 3.8GHz, 6GB RAM, NVIDEA GTX 970 4GB)), I get an amazingly smooth simulation. Heavy AI or clouds will still have a significant effect, though.
Unfortunately, aircraft designed for FS9 now have major problems in P3D, in that 'clickspots' in the VC no longer work.
If, as you say, you want to use DM's aircraft then I would advise sticking with FSX (ASN and FSUIPC are compatible with this platform).
An alternative, is FSX-SE (Steam Edition). Basically, Microsoft sold FSX to Dovetail games and this is now marketed through Steam. I believe that some improvements have been made, but anything compatible with FSX should work with FSX-SE.
Dovetail games have a new flight simulator due for release later this year. I can only assume that it's based on FSX, but what it's backward compatibility will be is anyone's guess.
What are your new PC's specs ?
Whatever your choice, happy simming
Regards
Kevin