i wouldn't drill the bottom of the tank unless you plan on installing a water tight overflow box around it. if the bulkhead ever leaks, you'll drain the whole tank empty.
as long as you use a pump that isn't pushing the drain to capacity, you won't hear much of anything. right now i have the mag 9.5 running on my tank and i do have standpipes, but nothing on top of them for noise reduction, and i can't hear anything. if i increased the flow, that's when i'd hear the suction toilet flushing sound.