I'm with patio.
honestly, PSU at top or on bottom, the changes/benefits in airflow would be so small that they'd be negligible.
they have only moved to the bottom in the last, say 3-5 years, it's just a current design trend more than anything else.
in fact, one design flaw of the bottom mounted PSU is their insane ability to suck up dust and fibres.
your choices of fans will not be limited at all.
you have options for front, top, rear and side fans.
and the size of those holes for mounting fans look like 72, 80 or maybe 90mm, which are all readily available. (as are any other size)