The server use case for Mac Mini is a huge compromise. Sonnet Tech [1] makes a 1U rack enclosure with a PCIe riser connected internally to the Mini's Thunderbolt port.
Internally we had one used for our company's OSX Server running profile manager before going to Jamf Pro.
Ok. But then, which Mac would you use as a build server? Mac servers no longer seem to exist, iMacs and Mac Pros are not stackable, you can't build iOS/OSX apps on a standard linux box, and laptops have the same issue as the Mini. That's why a Pro-focused Mac Mini, with even a mid-end desktop CPU and reasonable RAM would be ideal here, and a Mini with a modern laptop CPU would still be a vast improvement of the current situation.
I'd say this use case is ideally suited to the Hackintosh but I haven't attempted it. I mean there's all that server grade hardware out there just waiting to be hacked to work with OS X.