To round off this thread I've now built my computer and it's still working 10 days later.
The parts. The graphics card is an Nvidia GTX960 which I put in my old computer last year in an attempt to improve FSX frame rates. It had practically no effect.
Motherboard in. The M.2 250Gb hard drive is the small card near the bottom of the motherboard with '960 EVO' written on it. You may notice that at this point I've put the RAM modules in the wrong slots. I originally thought they should go in slots 1 and 3 but after checking discovered that slots 2 and 4 are correct for two RAM modules.
All done. There's plenty of space for air to circulate mainly due to being able to get a lot of the cables threaded through the back of the case and because I used a semi-modular PSU so unlike my older computer there aren't a lot of unnecessary cables present. Spending more money on a better quality power supply is one piece of advice that I saw regularly and I'm glad I didn't buy a cheaper option.
This shows the temperatures of the motherboard and CPU which given that the room temperature was over 30°C is pretty good. All of the fans are turning slowly which leads to one of the nicest improvements with this computer, it's almost silent.

The old computer sounded like a VC10 even before starting FSX and would regularly shut down due to overheating during the summer.
All in all the actual building of the computer was easy. The hard part is choosing the parts to get a combination that works well together. Once I'd decided to go for an i7 CPU that meant getting a motherboard/chipset and RAM to match. The idea was to have a computer that can be upgraded with more RAM a better graphics card and more drive space in the future.
Thanks again for the advice.
Jon