I am using the B550 Steel Legend (6 full speed SATA ports, which is perfect for raidz2 in ZFS), a Ryzen 5800x (you can't use the g variants with Unbuffered ECC)

I am using the B550 Steel Legend (6 full speed SATA ports, which is perfect for raidz2 in ZFS), a Ryzen 5800x (you can't use the g variants with Unbuffered ECC), and my RAM is Kingston ECC Unbuffered DDR4 3200 Model KSM32ES8/16ME. You will need a dedicated graphics card, thankfully they are going down in price, if you are going to run Plex/any kind of transcoding, I recommend looking up "NVENC Encoder Matrix" and finding a card that support h265 and B-Frames (Turing or later). Biggest cost is easily the CPU, a 5600x will run you $190. You could try a 3600 for $150 but I have not tested that with the ECC RAM, which is completely undocumented what kind of support is there it seems. You probably want the 5600x anyways, I was able to overclock the ECC RAM up to 3866MHz and set my infinity fabric FCLK to 1933MHz (the 3600 can't reach that really at all). It's only like 5% more performance though. Might need to look at used parts (Mobo, CPU, GPU) to get it for $600, but I haven't looked up prices and tallied it up.