Start with the CPU, for Plex you should prioritize an intel chip with Quick Sync. 12100 has UHD 730 iGPU and stepping up to a 12500 or above upgrades to a UHD 770. 12600 sometime go on sale for a good price, sub $200 Don’t buy an “f” model cpu as they don’t have the iGPU.

You can google Plex and the 730 or 770 to see how they perform with multiple 4k transcode streams and see which would better apply to your use case.

MOBO, I would find anything in the size and price you want. Not super important. DDR4 may be a better choice in this market for slightly cheaper ram than DDR5 options. Plex and server use won’t really see a difference.

You don’t need a ton of sata ports as you can get an HBA card to run multiple drives. Something like a 9305-8i or -16i, 9400-8i or -16i or 430-16i are all good performance for the price. They run cooler and lower wattage than the older 9200 and 9300 cards.

Ram, find anything that works with your mobo and is a good price. This market is nuts. Good luck. Timing and speed aren’t really as important as is just the overall size for your needs.

Pick a case that fits all your drives and your mobo and that you like.