1) You can't unlock a Sprint S3 - because it can't be locked.
2) For wifi only, locked or unlocked (on phones that CAN be locked) is irrelevant. And LTE is redundant if you're not going to activate it - you're just paying for a capability that you're not going to use.
3) The price sounds good. You could buy a tablet for that price but it wouldn't be a very well-made one. Any phone with wifi capability (which means any phone made since Android came about) will work on wifi without activation. I use a few that way myself. (One that's "locked to" Sprint, too. [IOW, it's a Sprint phone that hasn't been activated in over a year, but still works fine on wifi.])