Internal upgrading isn't necessary anymore. These days, any Mac with Thunderbolt 2 or Thunderbolt 3 can get PCI-level speeds with an external connection. You could also expand the memory if you wanted. This PCI connection was vital if you wanted to boot from it, as any other connection like FireWire or USB would be too slow. Older servers like the original Mac Pro tower could be opened up easily, and had extra PCI slots for additional hard drives. Therefore, you want to future-proof your Mac server as much as you can.
Ideally, you'll want to replace that hardware as infrequently as possible. You'll use it for backups, file shares, hosting websites, and more. This is the device on which your entire environment will depend. One of the most important features to consider when choosing a Mac server is how much you can upgrade it. Let's take a look at what Macs make good servers, and which one might be right for you. These days, there are multiple Macs that can wear the server mantle, each with their own pros and cons. From the days of the first aqua green G4 tower, it has appeared in such shapes as the HAL-like Xserve array, the silver G5, and the expensive trash can aesthetic of the Mac Pro.
