In the App Store click on featured on bottom left. Scroll to the bottom. Click on Apple ID. Sign out, sign in or create new.
Keep in mind, if the phone ever needs to be re-installed you most probably going to need the old owners login as he didn't remove himself from the device.
The phone can be restored. If it has find my iPhone enabled, then Apple ID and password associated to that device needs to be entered to pass the activation lock screen.
Correct, as long you can access settings menu and do full reset. You don't need old user password, it is only required for newer devices.
EDIT: You don't even need to reset device. Just go to the "Settings" -> "iTunes & App Stores" and change to your new Apple ID. You can access new content, still having old downloaded apps installed.