i highly recommend 16 gigs of ram for fl studio and at least a half terabyte of storage, i only have 250 gigs and i can't store all my plugins 😂
if you're going to get two 4k monitors you'd be paying a lot more and i don't really think you need it, just 1440p would be enough imo
you'll also need at 2-3 hdmi ports, two for the monitors if you still want two and one for the cintiq
not sure how to make a good build but these are the things i personally recommend if you want a solid build with all your needs, and 1500 will most likely let you fulfill them