What Schwalbe Nobby Nic should I buy?
I would go with something in the 26x2.25 range with the Double-Defense protection.  The 2.25 will give you nice volume to allow lower pressures while still being narrow enough to fit most frames (you might have to go narrower if your frame won't allow the 2.25).

Double defense will give you the maximum protection, it weighs more, but I doubt you would notice unless racing.

Unless you are well versed in running tubeless, I would just stick with tubes.

Hope this helps.