My initial novice thought is a spillway that is not built properly (and used like an overflow pipe) will be alot more headaches and repairs than a overflow pipe that will give out in "X" number of years. A big question would be how often is the overflow pipe expected to be used? Will it be a constant flow thru or once a year?

I have seen some lakes/ponds, mostly gov't built, that have a spillway that is used non stop, like a common overflow pipe. But, most of us do not have the money to build it properly so that there is not yearly troubles with it.

Have you considerd a siphon drain?

Personally, the funds to do it right may be more than you thought intially. The dam is a permanant part that is needed now. Put the money into it now, if needed. The aeration can wait till you save some more up in a year or two. Not as crtical of factor like the dam. Wait and see what others will say on this.