You might want to look into getting an e30 bently. I think you defintly should check into the park brake, unfortunatly i don't know a lot about it. Too bad your in arizona and not michigan i would volunteer to drive it and let you know what I think, I am very familiar with the wheel bearing noise.
From your new stuff I would suggest looking into the Brakes a little further. Otherwise at the wheel bearings. Can you say if it makes more noise when taking a turn a certain direction? I know that when they go bad the wheel bearings are capable of being pretty loud.
As far as diff oil i thought you just fill it till it comes out and you should be okay, someone please correct me if I'm wrong.