Samsung Galaxy S3.. unable to install keyboard ondevice.

Hi, I bought a samsung galaxy s3 yesterday and the guy said, that when he cleared everything off his phone to get it ready to sell he must have deleted the texting app. We fiddled with it for a while and he tried to get it to work. I thought i could download it from playstore on a laptop and get it working that way..but it doesnt recognize the phone.
Also, Im now realizing theres a program called KTweaker (?) Installed that was not removed during a factory AND a hard reset. Whats going on? Is this phone fixable?

The guy gave you a phone he had previously rooted (hacked).