If expandability is not necessary, MiniPC is a good option. 
Yes, mini PC is very nice for business office use case. Cost is fair for fast CPU and lot of memory.
They are for office desk where load is short peak (open a document is 1 second peak load) or medium load for video conference or video playing.
Larger case with bigger fans are need for business server handling constant high load. May use same chip as the mini pc, just bigger heatsink, more fins and bigger fan, bigger case.
I am trying to assemble using case at 7 litres size using STX sized power supply. Intel CPU 65 watts (154W peak) TDP. Total 220 watts and ASUS advise using 550 watts power supply.
4 litres, holding as hardare, seem a bit tight and need FLEX s-ze power supply.