For the RAM, for that small of a difference, 3600 is fine. As for the CPU, I'd say that the benefits will only be noticeable when there's no framerate cap like 60 fps and every bit of CPU power will help get more frames. I say it's not necessary to get the 3600X, but if you don't mind spending the extra $25 rather than spending that on another part, then go for it. I'll surely say that the CPU cooler on the X is better than the 3600, so yea.
I think it's unlikely that you'll have so many games installed on the hard disk at the same time to fill up that drive.

But yea, I merely gave suggestions. But I think that with next gen consoles using SSD's too, I see the future of gaming being more optimized for SSDs, so you might start seeing the load times increase, or worse, texture pop-ins while playing a game cos texture data didn't get accessed and transferred fast enough.
6 GB VRAM is sufficient for 1080p gaming. You can easily max out your settings without running out of it. The 5700XT is mostly meant for gaming at 1440p resolution, so there's no point in getting it if you're only gaming at 1080p. That's like buying a Ferrari only to drive it at 60 mph. You might as well have got a Volvo.
Trust me, playing on a 144 Hz monitor is a HUGE game changer. Once you experience high-refresh rate/framerate gaming, you wouldn't be able to go back to 60 Hz and all console gaming will seem piss poor. I say, take the $130 that you save on the 5700XT and get a 144 Hz 1080p monitor. It's a MUCH better use of your money. Cos it absolutely makes NO sense to pair a 5700XT with a 1080p 60 Hz monitor.
For case fans, I think one intake fan up front and one exhaust fan at the back is a must. You need to have fresh air flowing in. I say yes, put 2 more fans up front and put the one that came with the case in the back. If you can't get 2 then minimum 1 up front. More airflow = lower speed of CPU & GPU fans = quieter system + better performance.
Most of Intel's current CPUs are based on an older architecture, which means it draws more power and produces more heat leading you to spend more on cooling the CPU. Not to mention that Intel CPUs and related motherboards are generally more expensive than AMD, reducing their value for money. But Intel CPUs are generally faster (though not by much), especially if you plan to overclock it. AMD's Ryzen CPUs are generally much more efficient due to much better architecture, so much more power efficient and provide more cores+threads than Intel for the same money. They're also slightly better for productivity work. I say that unless you REALLY want the fastest processor on the market and don't mind paying a huge premium for it, AMD CPUs are the way to do. The Ryzen 5 3600 or 3600X are great value for money CPUs for gaming. You can go for the 8 core-16 thread Ryzen 3700 or 3800 if you're gonna be using it for heavy CPU workloads and time is essential.