The more powerful hardware you use the more your electric bill will be. I would go with an Intel cpu to use Quick sync for your plex streams. It allows you to get away with using a less power hungry cpu. I would suggest any intel cpu that has a UHD 700 igpu. It will handle multiple 4k streams. The intel i5 12500 is a 65w cpu and can be bought new for $175 on Amazon US. The 12600k is on sale for $150 on Amazon US but is a 125w cpu. You can find better deals if you search /r/hardwareswap on reddit or search on ebay. Then grab the cheapest Z790 or B760 chipset motherboard that fits your needs/budget. With this CPU you can choose to get a motherboard that uses either DDR4 or DDR5.