Unfortunately 32GB dimms are extremely limited choice right now on market, basically just the Corsair ones.
You don't have to buy the single-set 4x32 2667 CL16, you can buy their 2x (2x32 3000 CL15 or 3200 CL16), but it's probably the very same Micron memory of slightly different binning.
They sell the 128GB kit rated for such slow speed as this is the only guaranteed stable speed of most boards and chips.
I am actually waiting for the very same memory, my local shop delayed the memory order (Corsair 2(2x32GB 3200 Cl16) until 6th December, so I cannot tell you at what speed they will be stable. I do hope to get them running at 2933 CL15/CL16.
With Zen it's slightly easier to get faster timings than frequency.
Of course, faster memory benefits Zen...but there isn't much choice at all. G.Skill has already announced faster 32GB Dimm kits, but without release date or price. I would expect January earliest. It is what it is.
Power supply, anything from reputable Vendor (Seasonic, BeQuite, Corsair,etc.) and ideally OEM maker (Seasonic, SuperFlower, CWT). Since Seasonic is always the creator behind their products, it's always the safest choice. You cannot go wrong with Seasonic.
Prime series Gold is good choice for budget oriented build. For single CPU + top GPU 850W is currently the best choice, but if you plan to use two GPUs, 1000W+ would be be future proof.
NVMe drives are worth it if you routinely transfer high amount of sequential data, like copying hundreds of GBs of photos,etc.. otherwise there is almost no performance difference. The random 4k read/write is barely perceptible improvement, software runs the same.
Not sure what you mean by cache memory (Ram drive? Swapping in general? Anyway, it's 100 times slower than DDR4 memory, so it's always terrible idea). Reading of large files (like Photoshop,etc..) isn't any different.
Since system drive doesn't need to be more than 1TB, getting NVMe drive like Samsung 970 EVO makes sense since the price different isn't dramatic, but for 2TB+ drives for you projects, assets, etc.. you should decide based on budget. Capacity > Performance.
Flat bed cases went out of fashion quite a long ago, but you can very much place any case horizontally.