Not so Socialized German Shepherd? What are your thoughts?

I did a meet and greet with a beautiful male German Shepherd this week. I spent about an hour with him and his owner. He was very protective of the owner but not aggressive and he was very responsive to the owners commands. He did bark at me quite a bit but he eventually seemed to come around as we were playing fetch in the living room and he would bring the ball to me. I will not say his bark was not intimidating but he is after all a guard dog. I am also taking into consideration that German shepherds in particular have a really bad rep; kind of the same rep that pit bulls have (I have watched a couple of pits bulls by the way but I was very sure that they were socialized and not aggressive).

So down to the point. This dog does not know me that well yet. I have no experience with this breed especially one that is not socialized (he is two years old btw). The owners just really want someone that he can become familiar with and bond and basically be a reliable sitter for him. They have a vacation set for spring break in March so there is lots of time to try things out. I really do still want to work with him so I am going by and see him a few more times (maybe a couple times a week) with the owners present and give this some time as he is not socialized. I really do not want to go by without the owners present as I am unsure of how he will react, I really want to give him the benefit of the doubt but do not want anything to happen! I know some dogs can get weird when their owners are not around. I told my sister about this and she was very worried and said that I shouldn’t be alone with him for a while. I talked to the owner and he is cool with us working on it. I am not an experienced dog trainer by any means( I have tons of experience and am good with animals), I am just a dog lover that loves taking care of animals! I really think everyone deserves a chance but maybe that optimistic thinking is flawed.
I guess my question is at what point do I know if I can do this or not, how much time do I give it? Do any of you have similar experiences?