Birds need a varied diet -- definitely not just seed! And yes, salt is bad for them. But unsalted sunflower seeds would make a good treat.
I know you have probably made up your mind to get the green cheek, and I hear they are great birds, but I personally would recommend a cockatiel. They're actually a tiny bit bigger than green cheeks, and so sweet and laid-back. Mind you, they all have their own individual personalities...but I have seven and I love them all to bits.
I've read that green cheeks can be a bit feistier, and go through a nippy or aggressive hormonal phase. They also have more of a tendency to be cage-territorial. And recently, I've read several stories from greenie owners lamenting that their bird has "turned on them" (again, onset of hormones). Not trying to totally discourage you, but I think they can be a little more challenging.
Another type of bird to look into might be a pionus. Not too big, very pretty, and I haven't read much negative about them.
Thanks. I was told that a budgie would be able to sit on my shoulder and talk, sing and whistle. In another thread I read that for macaws and other types of parrots, you should never let them sit on your shoulder without a strong trust bond because you can't see the signs it's about to bite. Is this the case with budgies, also. I'd be concerned about getting nailed in the ear or back of the neck.
Overall, though, I would say a cockatiel is the better bet if you're looking for a bird to pick up sounds.