Tl;Dr - more cores, best graphics you can afford now. High resolution (4K) is nice
The latest high-end sims are 64-bit and are coded to use every resource you can throw at it, including cores. The old FSX is just 32-bit and hammers one core, making a fast clock Intel best if you are going to sim with old software.
Also, the more addons, such as real world weather, traffic, complex aircraft and high detail scenery add to the CPU burden and need for cores.
For simming, SMOOTHNESS is more important than frame rate, especially when landing. This is where more cores help.
My (now old and lowly) Ryzen 1700 at 3.4 GHz with GTX1080 at 4K resolution has seen up to 95% CPU load on all 16 threads. I set a max framerate of 30 fps and have non-existant stutter - for the record, with a clean install of P3Dv4.4 and no addons, I've seen 240+ fps at 4K at cruise height, stock aircraft.
I watched a YouTube of a guy using P3Dv4 and 8700K crowing about his 60 fps, but he ignored the stutter, sometimes dropping to a momentary low of just 4 fps. Fast CPU but not enough cores. Stutter sucks, smoothness brings smiles.
Every serious simmer I know has the sim installed on its own SSD, with only sim stuff (aftermarket aircraft, scenery, weather, traffic programs etc). A motherboard with a second M.2 slot is an advantage, even if budget means you can't fill it now. I use a M.2 Samsung 960 Pro as boot drive and M.2 Intel 1 TB as sim drive. All my sim stuff currently fills 750 Gb.
You can work up to better hardware with upgrades over time eg. my sim used to be on a shared HDD, then shared SSD, then M.2 for only P3Dv4.
I have tried multi screens, but settled on a high quality Samsung 65" 4K TV for a wonderfully immersive experience, with yoke, throttle quadrant and rudder pedals. YMMV.
Flight simming at 4K is my main use for a high-end PC. Since 1985, and Bruce Artwick's flight sim, simming has driven all my hardware and software upgrades, to try and get more resolution and realism. I am joining the line for a high core Ryzen 3 and will wait and see where the graphics cards sort out, then.