You will not be doing any kind of mining with a SFF system. The PSU is too weak and there is not space for a video card.
You could do a frankenstein rig by moving the guts to another case or even an open system but you would still need a more powerful PSU to power the strain the CPU/GPUs would endure.
Back in the Bitcoin GPU mining days I used Dell SFF systems and used another PSU and GPU ribbon riser to mine. With the arrival of USB Risers and dedicated mining boards it just makes sense to build a new system unless you already know how to tinker with hardware.