Various articles I've read all point towards more lower speed RAM being preferable to less high speed RAM
Maybe for servers or other types of configurations. Or depending on what is meant by "more". Certainly 16GGB of DDR4 2666mhz memory is preferable to 4GB of 3200mhz memory, but once you get past the point where you have more than enough memory for what you are running, it doesn't make as much sense to add more of it as it does to increase the speed of it. And at 32 to 64GB of RAM, you are WELL past what you'd need to see a performance benefit for any gaming scenario I can think of, long term cache as discussed above, aside.
Also, you are COMPLETELY confusing what we call "overclocking" on modern memory modules and what is simply the most common of manual configurations which is to set the memory XMP profile in the BIOS. Setting the memory speed and timings to what they were designed for by the manufacturer, is not overclocking in any acceptable sense of the term. Technically, it might be because it is above what the JEDEC default configuration allows for on a given platform or chipset, but it is NOT overclocking in any acceptable sense of the term. There is no requirement for stability or thermal testing involved with setting sticks to the XMP profile. The manufacturer has already EXTENSIVELY done that themselves to ensure that a given kit runs without issue for the broadest number of systems at it's profile configuration. Obviously, for chipsets that don't support overclocking, then XMP profiles that are beyond the default speed supported for that chipset typically can't be used at the profile speed, but for the chipset and CPU you are planning to use there should be nothing more than changing a single setting required to run at anything up to probably 3600mhz with two sticks installed. So, you can do what you want there, but NOT using speeds above 3000mhz on this platform is intentionally, and unnecessarily, leaving performance on the table for no good reason.
The fact that you DO do professional work and work with files that will benefit from it, makes it entirely understandable that you would want more memory. Since the stress on the internal memory controller increases based on the more sticks you add, I'd recommend achieving your desired memory capacity using as few sticks as possible, especially since this is a dual channel platform, not quad or higher channel chipset.
In that light, I think this might actually NOT be the right platform for you to consider. You'd be better served using a platform with quad or six channel architecture, but obviously that's just a preference, which also gives you the option of increasing your total memory capacity later if you find the need. Putting that aside as an option, it would seem you ARE going to require four DIMMs to achieve 64GB as 16GB DIMMs are the largest common option available currently for this platform. Having four DIMMs shouldn't affect being able to set a quad set of sticks to the XMP profile speed, say, 3200mhz, considering the already high clock speed and strength of the memory controller with this chipset and CPU. It COULD require needing to bump the DRAM voltage up slightly in order to get them to run, but two small settings seems a small price to pay in terms of extra configuration steps to gain a modest bump in performance. Either way is fine though. You're probably not going to lose any sleep over running with 2666mhz sticks if you go that route.
The Noctua NH-D14 is only 162mm tall, AND if you do THIS, it outperforms the D15 anyhow. You will however want to either focus on getting memory with very short heatsinks OR move the front fan to the back for a pull-push-pull configuration (IF the case will support a rear fan with that heatsink installed) rather than a push-pull-push configuration. There should be no appreciable difference in performance regardless of which configuration you choose.
Various combinations of fans were tried on the NH-D14, especially the NF-A15 and the NF-A14. We found a winner.
www.overclockers.com
Otherwise, I think you'll want a new case because none of the big air coolers are going to fit the constraints of that one, and I don't know of any other cooler which is smaller that I'd recommend. There are certainly a lot of coolers that will technically "work", but they are going be loud and annoying, constantly ramping up and down every time a core load is applied or simply running full speed to try and keep up with an all core boost load. If you can't go with one of the biggest air coolers, then I'd seriously consider a high quality large AIO, or a different case.
Graphics card, that's the right one. There really IS no other choice at the resolution you desire.
Cases, you might need to make concessions here if you decide to replace that one, by either foregoing the idea that it has to be metal or the idea that you can get everything you want in a case that costs less than the motherboard.