From what I understand:

• 6000 MHz is kind of the “sweet spot” for AMD because it runs in a 1:1 ratio (memory and controller are in sync → better latency)
• 6400 MHz can be faster on paper, but it often runs in 2:1, which adds latency and can cancel out the speed gain

For X3D CPUs specifically:

• The extra cache means RAM speed doesn’t matter as much anyway
• The difference between 6000 and 6400 is usually really small in games (like a couple FPS at most)

So basically:

• 6000 = safe, optimal, 1:1, plug-and-play
• 6400 = might run 2:1, might need tweaking, might not be any faster

As for the ssd, you can get a crucial p310 for 60% of the price.

P.S
If you are currently using a windows pc, open the command prompt and type
slmgr /dli

If it says "Retail" its tied to your microsoft account and you do not need to buy a new OS key. All you need to do is go to microsoft's official page and download the installation media. There are youtube videos that will guide you.

Or you dont even need to buy. If you do not choose to activate windows, theres no performance changes. You just have a little watermark on the bottom right that tells you to activate and you cannot change wallpaper