So as I understand from reading your post, these are your requirements: IPMI, lots of PCIe slots, cores for VM's, and somewhat power consumption conscious...correct?
If that's the case, then your really have two paths that aren't going to cost an arm and a leg (so no Xeon Scalable and AMD Epyc) and give you the PCIE slots you want (so no Xeon E and AMD Ryzen). The two paths you are left with are LGA2011-3 Xeon (older but has USB 3.0, DDR4 ECC, and is becoming cheaper very quickly) and AMD Threadripper. Here are prospective choices with those paths.
- CPU: If you want VM's or lots of Docker containers running, then you need to go with the E5-26XX v3 series and probably not the E5-16XX v3 series (they have less cores but higher frequency and single thread rating). I'd stick with a single CPU since you're wanting to go lower in power consumption and it will save you money in both motherboard and CPU cost. The E5-2640 v3 is a really nice lower power CPU (it will idle much lower than its 90W TDP) to use for you. It has 8 cores/16 threads, 13,864 Passmark, and can be found Used on eBay for around $75. You could also go up to the E5-2680 v3 (18,426 Passmark, 120W TDP, 12 cores/24 threads, $150) and the E5-2690 v3 (19,240 Passmark, 135W TDP, 12 cores/24 threads, $200) if you want more power. Not to mention that any motherboard that can have a v3 processor can also take a v4 one. The E5-26XX v4 is still really expensive but these will decrease over time and give you a decent upgrade path. **Side Note: Don't buy QS/ES samples or ones from China. There are plenty of good, regular, used ones around so don't be drawn in by slightly lower costs.**
- Motherboard: The X10SRL-F is pretty much the standard. It has a ton of PCIE lanes, USB 3.0, IPMI, fantastic support, and plenty of used ones on eBay. This seller: https://www.ebay.com/itm/SuperMicro-X10SRL-F-ATX-Single-Socket-LGA2011-v3-DDR4-Motherboard-X10/123818059106?hash=item1cd421a562:g:n3sAAOSwwrlckRA- regularly takes around $60 lower for a one than what is listed. The I/O plate can be found on Supermicro's website for only a couple of dollars. If you want to buy it new, Provantage has them for around $260 which will also come with a warranty which is nice. Just beware that if you pick this board, you'll need to find a heatsink that fits the LGA2011-3 Narrow socket. Noctua has a 3U one that would be perfect for your Supermicro chassis (I actually have the exact chassis as you!).
- RAM: DDR4 ECC RDIMM's are dropping considerably in price. If you look on the Used market (eBay, ServeTheHome, or r/homelabsales), you can get some great deals. Hard to say what you'll pay because you didn't specify how much RAM you were planning on using. If you're going to do VM's and Docker containers, more is better. Also, take into account that this is a Quad Channel CPU so you'll want to use at least 4 sticks to really get your money's worth.
- CPU: The 1900X is actually not a bad choice if you're planning on getting started with this setup but don't want to spend a bunch of money. It has 8 cores/16 threads, 16,108 Passmark, and can be found for about $150. You could also go with a 1920X ($250) if you want more cores. All threadripper CPU's will come with a higher idle power consumption though (starting at 180W TDP with an idle around 100W). You do, however, get higher Single Thread Ratings and a bigger upgrade path.
- Motherboard: If you want IPMI, this leaves you with exactly one choice: ASRock Rack X399D8A-2T. This one is actually pretty hard to find and is damn expensive. It costs about $550 (I know! Crazy right?!) and it can be found on eBay and a couple of other sites. NewEgg is out right now but it'll probably come back relatively soon. Although it costs a ton, this motherboard has everything. PCIE lanes, IPMI, 10Gig, you name it.
- RAM: DDR4 ECC UDIMM's are starting to go down in price and Provantage has some nice 2666mhz Kingston ram sticks for a decent price. You can also do non-ECC dimms and get better speeds if ECC isn't important to you. The only issue with Threadripper and RAM is that it really likes to have overclocked higher speeds to get the most out of the CPU. Since this is a NAS we're talking about, I don't condone overclocking ram but some people on here think it's worth it for the performance boost.
You can build a damn nice LGA2011-3 build for a lot lower cost than a Threadripper build while only suffering a little in high end performance.
**Personally, I really want to do a LGA2011-3 build but I just don't have the need for all those PCIE lanes so I'm probably going to go AMD Ryzen instead.**