It will be an overhead filter wet/dry filter. So it's probably not a good decision to make one with my circumstances right?
Actually, it's probably the best approach as long as the power is not out long enough for the media to dry out completely. And you can manually wet it if needs be while the power is out.
Wet media that has access to aerial O2 can deal with massive amounts of nitrogenous waste because the only thing limiting filters is oxygen, hence why wet/dry filters are so much more efficient in dealing with ammonia/nitrites even with a way smaller volume of media.
Stop an external filter and it can completely de-oxygenates itself given enough time. You'll soon smell the sulphur if you open it afterwards because anaerobic bacteria takes over as the aerobic such as nitrifying bacteria simply can't function anymore. This doesn't happen in a trickle filter for example.
If you have an external filter and the power stops, you need to open it, empty it from water and just keep the media moist. The bacteria will then survive just perfectly.
I don't know why people suggest submering the media in water that doesn't get oxygen replenished during a power cut, It's the worst you can do for both the media and the fish as the bacteria in the media will keep draining oxygen from the water when the fish need it the most.
So I think a wet/dry filter is the best idea actually...