iPhone 5C Water Damaged - Won't turn on - How to troubleshoot?

I have a iPhone 5C I purchased cheaply off a classified

It won't turn turn on, and the water damage stickers have been activated.

When I plug it into the wall, the screen will also not illuminate... I plan on just reselling the phone, but is there any point in purchasing a replacement battery to try? Or does plugging it into the wall totally override the battery and hence no point?

Anything else I can try? or will it most likely be dead?

Cheers