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
- casaos: easy
You can get pretty sleek mini-pc in the price but, there performance is sub-par compared to the price they are asking for.
While itx for factor is very expensive, m-atx is cheaper and there are quite few sleek cases for them.
If you are looking for prebuilt mini pc, asustor flashtor is pretty sleek if you only want m.2 storage.