In general: Even the slightest airflow will cool your heatsinks down.
I did a test with a very precise thermometer attached to a heatsink. Even when I walked by, I saw the numbers go down. Not a lot of course, but still.
So with a fan it will work even better.
Another (and imo an even more important) benefit is, that with having a fan above your led pushing the air down, is that more heat will get to your canopy.
Where with HPS you want to suck away the heat, with led we sometimes want the heat to go to our leaves to reach the best temperature.
If you want to know if only a passive heatsink will do, you can use this
calculator.
You said you have plenty of time, so perhaps you can figure it out.
I once found a much simpeler calculation somewhere, but can't find it anymore. I was something like: If you want to cool something 1 degree down, you need x cm2 of aluminium surface. It was over simplified of course, but it was a handy tool.