You can definitely get PCIe on some micro PCs. I have a Lenovo m920q that I use with a Mellanox NIC as my router.

You could certainly install a SAS or SATA controller, the issue would be having somewhere to mount the drives, and a way to power them. External SAS enclosures are not cheap.

M.2 SATA cards are also a thing, I repurposed a NUC in a SuperMicro (SYS-521R-T) mini tower server with 4 drives and it works great.