Regarding FPS: as this is a flight simulation, you do not need very high frame rates. Anything consistently above 30FPS is good enough. The key word is consistent: when looking at benchmarks, watch on 0.1 and 1% low rather than averages as this is what will determine smoothness.
When selecting a GPU, pay great attention to the amount of VRAM.
Generally 8Gb is good for 1080p, maybe 1440p not more.
For 1440p it is best to have 16Gb, although 12Gb may suffice depending on usage.
For 4K a minimum of 16Gb is preferred, even higher if possible.
If using DX12 and the more you indulge in addons, the higher VRAM usage.
Regarding CPU: the smoothness this game is very much a function by the CPU performance. IMHO, you should get the highest performance CPU you can afford, than