I have an old hp dual core laptop running Ubuntu with pihole on it 24/7. Battery is in, plugged in, with screen open and on 24/7 - 365. It’s been fine now for over a year, i upgrade the software once every few months.

Are you specifically looking for SFF drive bays? Because my one recommendation about buying a refurbished server is get LFF 3.5 drives. The sff is such a limiting factor.

I used a r710 when I first started homelabbing. The cost after six months of just powering it was equivalent to a completely separate server upgrade. Even a r720 is a massive improvement over the 710. Do yourself a favor spend a little more to start and your future self will thank you.