I don't know the codes in your area, but I do know the ones where I live are very strict. My furnace sits on small rubber feet about a half inch thick. You may want to raise the furnace if your basement is prone to flooding.

I'm not sure. Mine which is an upflow unit on a slab that is at grade, is sitting on an air box where the return air connects. Return air comes down from ceiling, attaches to box, furnace is 1 inch to the right, filter at bottom blower unit.

My parents is essentially the same, though located below grade. Sitting directly on the concrete seems to make it prone to corrosion.

Cinder blocks.