Talking about the return classes though, I normally create a set of classes to use as Json I found that makes it easier for when you want to send data to your controller and deserialize it.

Also, with your linq query, you can add a query to the SingleOrDefault Function so instead of .Where(x => x.ID == newID).SingleOrDefault() you can just use .SingleOrDefault(x => x.ID == newID) or alternatively you could add a IEnumerable Find(Func exp); to your IRepository interface and that returns GetTable.Where(exp);

LINQ is awesome once you get the hang of it! Keep up the good work.