I just recently started my journey setting up Plex with *Arr and have had a blast. I have the setup running on a raspberry Pi. Before I start buying a bunch of external hard drives, I went searching for some dedicated server hardware to comparison shop. Am I crazy to consider buying an old tower server for this, or will a raspberry Pi work just fine for this purpose? I don’t have that much experience but I do enjoy a good challenge.
I was using raspberry pi4 4gb with full arr stack + many more services in docker containers. Its not snappy, but it works fine. Transcoding is imposible, but with *arr stack its easy to chose quality and dodge transcoding and that was enough for me. Super low power consuption and 0 noise is amazing. But there is another problem, rpi doesnt have a sata port. I was using usb to sata adapter at first, it was perfect untill i upgraded SSD which needed more juice. That forced me to get sata dock with external power supply. Everything became bulky and not cheap at anymore. After that I got used PC with no GPU. Intel g3930 and 8 gb ddr4 is draining 25W on idle and its rock solid. It can transcode multiple 1080p at once. I still use rpi as a backup pihole and few more services.
I highly recommend getting something with sata or m2 ports, probably any intel gen 7 or newer with quicksync. DIY or SFF PC, new or used, doesnt matter.