First off, you don't need to take a tank down to drill it, I have drilled a number of them, almost full with fish inside, and also empty.
The process is the same.
Just drop the water level to where the holes are to be.
I usually drill just a few inches below the rim, and drill inside to out, using a hose for cooling water for the diamond bit, with hose water flowing into the tank, over the bit and place for the hole.
But
if you're not comfortable drilling that's a different matter. Or if your tank is made of tempered glass, and can't be drilled.
As far as overhead sumps go, I have done many. Some totally overhead, some half way between.
I just use small tanks and drill them, put them in window boxes, or just elevated, because I like to plant them heavily to help mitigate nitrates.
Below is a commercial type sump that came with a tank I bought used, it is elevated so after water passes thru mechanical and bio media, and empties into a 125 gallon heavily planted sump/refugium.

Below a long shot of the refugium the small sump empties into.
