Why not build a new PC or buy an old pc? One with ryzen 5 5600G, 8gb Ram, 250GB ssd should cost ~250USD whether you buy new/old, i recently checked the prices, cause I needed one and they were similar. This should take you a long way. As for storage just pick a case with enough ssd/harddisk slots.
You can also go much cheaper depending on what you get.
The advantage is you can add a GPU like the intel A380 for av1 encoding of video if you feel like you need it.
For OS depending on what you are doing there are a few choices:
True nas: mid difficulty
debian: i just use this with docker, does the job
alma linux/Rocky linux
ubuntu
yunohost: if you would be interested in hosting activitypub apps, easy