I would stay away from those shops in the upper floors. Don't ask me why but that's my suggestion. TCW is a decent place to go to for second hand stuff.
The BP-300 is not only a battery case. It also provide a set of vertical controls (shutter button, main dial, etc) which is extremely useful if you shoot in vertical format also. However, I didn't really like the feel of the grip. Too small and too shallow for me. Ironically, with the grip on, shooting horizontal is more comfortable.
Eye control. Like I said, it depends on what you shoot. if you're uncertain but can afford it, then by all means go for it. Bear in mind, it seems that eye control is only useful to some photographers. It doesn't help if you're wearing glasses according to a friend who wears glasses. Many photographers get by with no eye control on so many cameras. Useful or not, only you know.