Reusing power supply?

REUSE IT.

You are obviously building a budget PC anyway, so go ahead and reuse the power supply.

(However if you don't have the APU yet then don't buy it. It's rather pointless to get an APU and R9-270 since you won't use the GPU in the APU you're wasting money. Better to get a CPU instead though an i3-4160 setup would be a much better choice IMO)