Been doing some more looking and I'm struggling a bit to find something else as I'm not a psu expert and I only know that the xigmatek Asgard case is ok at £26 as my dad has one.
A gtx760 is about £160 and an r9 280 about the same.
Once you throw in a psu is it worth the money when a 750ti will max train sim and be a good match for his cpu in fsx, especially without addons.
To give you an idea, fsx only needs a beefy gpu when you have high AA in lots of clouds (where amd cards struggle against nvidia). My card hardly hits 50% usage with 4x dsr normally and I have lots of add ons.
If it was my dad I would be advising my dad to go down the 750ti route to be honest.
Then you can pick a nice case and cooler.