Depending on the overflow box that you choose will determine how you have your setup....setup. A CPR overflow box (thats the name brand) has an airline hole on the top in which you have to put a powerhead attached to the overflow airline spout heres a link for a pic of reference:
To draw the water out of the syphon i would recommend a rio 600SVT. These work better than other PH
since they draw the air from the intake not the output of the PH
. A built in overflow box will be far more better and easier to maintain. I have both so you'll have to take my word on that. A built in overflow you wont have to worry about a syphon breaking etc.
You are correct on the situation of drilling a hole on the return to break the syphon of flow back down to the sump. What you will want to do is drill a 1/8" hole just below the water line for this to work. Otherwise what happens when the power goes out is since the return is a "pressured line" from the PH
pushing the water back up it will begin to syphon the water back down until the syphon breaks. Then your sump has the possibility of overflowing!
Thats not ever fun! Make sure you test your setup every so often to make sure the water that does syphon down to the sump will not overflow your system.
Glad to help!