Video encoding you've really got 2 clear options: Either a 8th gen or newer consumer Intel chip with integrated graphics for QuickSync support or toss a GPU in there. You can also rely on raw CPU cycles for video transcode but that's wildly energy inefficient in comparison.

I've heard good things about how anything AM4 compares to x99 era Intel on both raw performance and performance per watt, but I have no personal anecdata to share.

Personally I'm currently eyeing up a gaming computer refresh as the opportunity to refresh my primary server with the old components from the gaming computer, but I'm also starting with literal ewaste I scrounged for free, so pretty much anything is big upgrade.

If you're on a budget, check out X99 socket Xeons. You can pick up Mobos and chips for super cheap. 10+ core hyper-threaded Xeons with solid clocks and a motherboard for 120-180 bucks total. Support 64 GB of RAM, more if you have a proper server board.

For transcoding, depending on the codec, dedicated GPU is best.

I'm not sure about Plex, but I know on Jellyfin, the new Intel Arc GPUs are really great for encoding, not too expensive for the lower end cards either, and low profile options for smaller rack cases.

I see a lot of drives there, all presumably connected via SATA. If you're looking to add more drives in the future I recommend a SAS card or two, specifically a Dell PERC H310 flashed in IT mode. I picked one up on eBay for $20 a while back and it gives me 8 drive connectivity. Also snag some mini SAS to SATA cables to connect the drives.

I've got 44TB running in my Plex server using it and have had 0 issues with the card. Even had a friend 3D print a fan housing and attached a small Noctua fan to the heatsink for some peace of mind when doing large data transfers to make sure the card doesn't overheat.

Edit: Like so

I think about this a lot and it really does depend on your needs.

Home lab vs home server. I like to keep them separate just because I consider my lab unstable and my home server stable. You don't have to do it this way it's just the way I like it.

https://a.co/d/6k6QpOD If you want to build a low power NAS I suggest investing in an Intel n100 based itx Nas motherboard. You can then use a case like this from Jonsbo https://a.co/d/1ayqwJV. This could be a nice cool and quiet solution. If you want to do video transcoding, the n100 has quicksync on board and with something like Truenas it's pretty easy to set up via the app catalog (check out truecharts).

If you want something even more simple (good for home users or like a backup target you keep elsewhere) I've been meaning to grab one of these "Topton 2-Bay NAS R1 PRO 12th Gen Intel N100 Network Attached Storage Media Server" from AliExpress for just this case.

As for a Lab, I suggest finding a W680 chipset based motherboard like an ASRock IMB-X1314 LGA 1700 Intel W680. You can get a cpu like a 12400 or 12500T (lower power and less heat) used cheap and you have the option to upgrade and use ecc memory without a XEON. You also have a lot more pci express connectivity.

What ever you do choose, anything pre 12th gen Intel is basically ewaste (those 11th gen mobile erying I9 engineering samples are very good but less reliable than desired). Do not invest in any old x99 based gear (unless you get it for free). I have an old dual XEON system that is still running and it uses power like a small fridge.

https://www.asus.com/us/supportonly/ipmi%20expansion%20card/helpdesk_download/

Get rid of that molex to sata adaptor, they catch fire. Molex to sata = lose your data

my 2 cents just in case...:

A raid6 is not a replacement for backup ;-) i use rdiff-backup which is easy to use, stores only one full backup and all increments are to the past while it is only possible to delete the oldest increments (afaik no "merging") i never needed anything else. The backup should be one off-site and another one offline to be synced once in a while manually. Make complete dumps (including triggers, etc) from databases before doing the backup ;-)

i like to have a recreateable server setup, like setting it up manually, then putting everything i did into ansilbe, try to recreate a "spare" server using ansible and the backup, test everything and you can be sure you also have "documented" your setup to a good degree.

for hardware i do not have much assumptions about performance (until it hits me), but an always-running in-house server should better safe power (i learned this the costly way). it is possible to turn cpu's off and run only on one cpu with only a reduced freq in times without performance needs, that could help a bit, at least it would feel good to do so while turning cpu's on again + set higher frequency is quick and can be easily scripted.

hard drives: make sure you buy 24/7, they are usually way more hassle-free than the consumer grades and likely "only" cost double the price. i would always place the system on SSD but always as raid1 (not raid6), while the "other" could then maybe be a magnetic one set to write-mostly.

as i do not buy "server" hardware for my home server, i always buy the components twice when i change something, so that i would have the spare parts ready at hand when i need it. running a server for 5+ years often ends up in not beeing able to buy the same again, and then you have to first search what you want, order, test, maybe send back as it might not fit... instable memory? mainboard released smoke signs? with spare parts at hand, a matter of minutes! only thing i am missing with my consumer grade home server hardware is ecc ram :-/

for cooling i like to use a 12cm fan and only power it with 5v (instead of the 12v it wants) so that it runs smoothly slow and nearly as silent as a passive only cooling, but heat does not build up in the summer. do not forget to clean the dust once in a while... i never had a 5v powered 12V-12cm fan that had any problems with the bearings and i think one of them ran for over a decade. i think the 12volt fans last longer with 5v, but no warranty from me ;-)

even with headless i like to have a quick way at hand to get to a console in case of network might not be working. i once used a serial cable and my notebook, then a small monitor/keyboard, now i use pikvm and could look to my servers physical console from my mobile phone (but would need ssl client certificate and TOTP to do so) but this involves network, i know XD

you likely want smart monitoring and once in a while run memtest.

for servers i also like to have some monitoring that could push a message to my phone somehow for some foreseeable conditions that i would like to handle manually.

debsums, logcheck logwatch and fail2ban are also worth looking at depending on what you want.

also after updating packages, have a look at lsof | egrep "DEL|deleted" to see what programs need a simple restart to really use libraries that have been updated. so reboots only for newer kernels.

ok this is more than 2 cents, maybe 5. never mind

hope these ideas help a bit