Not all tanks can be drilled. If the tank says "do not drill" or some such thing then it's tempered and can't be drilled. Unfortunately that us usually on the bottom I believe. Or the back glass might have it.
As for the sump, if you make it so the display tank level leaves enough room for the entire return section of the tank to be added or pumped up, the there will NOT be an overflow in the event of a siphon loss. Conversely you have to have enough extra room room the sump for a power loss. In both cases catastrophe can easily be avoided with a tiny bit of planning. Not only should that not worry you, it should never ever be an issue.
A drilled tank is always the preference, but if that is not possible or you do not want to drain your tank and put everybody in temporary lodgings, then you can certainly use a hob
overflow. Many members are doing that very effectively.