Almost any standard ATX unit will work, you just have to be sure to not purchase one that is to deep for the box.
The Dell psu is 140 cm, you could go up to 160 cm without problems.
Also the Dell psu is taller than standard, so if an aftermarket psu is installed there will be about a 1.7 cm gap below it, easily covered by tape or left alone, as it create's no issue's.
Do yourself a favor and pick up a couple of 80 mm fans to place on the rear grill, they will help keep temps down.