I've had a lot of fun with used HP elitedesk/prodesks. Go for Intel, gen 6 i3/i5 or newer. Id push it to 16GB ram if you can manage it.
The SFF models (small desktops) are good if you plan on adding HDDs or anything PCIe in the future. If not go for the USSF (mini) models - they are very power efficient. All extremely quiet - you'll have to really push the resources to hear more than a whisper from them.
I've had 7 all together and been pleased with them.