Hmm that hole configuration has me stumped. Looks like it was previously set up with a sump as well as some kind of closed loop.
I can only tell you what I would do, which would be an almost complete reconfiguration of the tank. I would plug all the holes on the bottom of the tank, and drill a few more holes on the back panel for a custom overflow setup. I prefer back or side panel drilling to maximize the space inside the tank.
I would drill 3 holes for 1.5" drains (2 main drains,, 1 emergency drain) arranged in the center of the back panel, possibly utilizing an internal coast to coast overflow box 30-36" long. I would utilize that one hole on the back with the strainer (looks to be a 1" bulkhead) for one return line, and drill another on the opposite side of corresponding size for another return line.
As far as sump size and configuration...very open ended. I prefer large, open, simple sumps for media storage, ease of access, and water volume. But you could go with an off-the-shelf wet-dry setup, custom made setup with baffled compartments, or even dual sumps if it worked to your advantage.
Imo, its not a question of what should you do, but what can't you do? The options are almost limitless for you at this time in my mind. Excited for you and anxious to see how you proceed.
