What unfocused said.
Plus, while I have a gripped 5d3 and love the grip, why are you needing a grip? Or is it because you want the dual batteries so you don't have to change in the middle of a field somewhere?
As for the 6D, it's a great camera from everything I've heard. The center AF point is very good, although the rest are meh.
As for memory card, from what I recall, 6D supports up to UHS-1. So pick a card that has UHS-1 support, and then pick you're preferred brand & speed.
Last bit, landscapes. You can do them without a tripod, depending on your style, but if you are going to end up being rather serious about them you should definitely consider a good quality tripod, which will run you $500-600 and up. But, if you just sorta have an interest, leave that for another time. Also get a cable release and learn about mirror lockup.