I had to reinstate my own server a couple of years ago after the old one died. Mine serves as source control, bug tracking, TV recording and media, as well as backing up the other machines. I used an HP Proliant Micro server, which are decently built and often on offer, think I paid about £120 after cashback. Don’t see any such good offers at the moment, but haven’t looked that hard. Reasonably compact too. Only space for four internal HDDs, though, do you really need more?
Other things to consider are...
If you’ll be leaving it on 24/7, consider power consumption. This might influence your choice of CPU as well. HP Micro seemed acceptable (to me) for power usage iirc, but can’t remember the numbers.
If you’ll be swithing on only when needed, start up time (BIOS self tests) before even starting to boot Linux, can be a nuisance. HP was not so good in this respect, probably just a couple of minutes, but that can seem a long time when you’re waiting to do something.
Do you need an optical drive? Don’t assume. The HP came without one, and adding it would have been a hassle, requiring some kind of special mounting kit.