Should you decide to run both those 3.5's in there I'd be sure to keep the dust out of the power supply and the CPU fan/heatsink assembly.....all those SFF PC's, both Dell and HP, can be hotboxes and depend heavily on all fans and heatsinks working well. Dust or other restrictions will impede that and we all know heat (next to shock) is a HDD's worst enemy, plus it can cause the CPU to throttle and slow down to compensate.
In the half-dozen SFF's I've built (4 are HP's) I ran a 2.5" Samsung 870 EVO SSD for the O/S and
Blue Iris VMS and a 4TB WD Purple HDD for video clips. It sure created some room for the PC to "breathe" better.