Dropped iPhone 5 reboot loop

My daughter dropped her iPhone 5. Before dropping it, it worked fine except the power button was tricky - had to toggle it backward and down to make it work. After dropping it, when plugged into the charger or a PC, it would show the low battery picture, then the white screen with black apple, then black screen, then repeat with low battery picture.

Probably unrealistically hoping the problem could be solved through software, we plugged it into the PC and got it to the connect to iTunes screen. Tried several times to first update it, no luck. Went to restore. The process will go through extracting software, but when it finishes that, the phone blinks off, interrupting the process. iTunes will continue trying for a couple of seconds in "Waiting for iPhone," but then says the phone could not be restored. Error 21 has come up then. Repeating the process and immediately pushing the home button after it blinks off, bringing it back to the connect to iTunes screen doesn't help - it still won't update. We've gone through this process many, many times using different cables, different PCs, different methods (home button alone, home + power button), etc.

There seems to be something happening at that exact moment in the update process that flips the phone off. When simply plugged and in and on the connect to iTunes screen, it stays on much longer.

So, logically, it seems to be a hardware issue caused by the drop. What is the most likely issue? Something simple like a loose battery? Something else? Since the phone is second-hand with no warranty, I'm inclined to try to open it up and see what's what. What should I look for?

If it can't be fixed, is there any way to remove her information so that it can be sold for parts? I know we can take the iCloud lock off, but what about her data? It seems that someone more sophisticated with iPhones could get into that.