So yeah, space will be an issue if you would go small... Some SSF may be enough but you will mostly need some special cables though. I got an SSF thinkcentre and managed to put 3x 2,5" HDDs inside (needed some machining).
Basically I would recommend something like an i5-9400 with 16gb ram in an tower case. Also have the PSU in mind, because some office PCs come with really low wattage, that could limit your expandability.
If you really want to stay with your mentioned software, 8GB could be enough though.