Hand grip? You can get the Nikon AH-4 or the Hakuba handgrip which sykestang is doing an MO for.
Yes, you should get the optech neck strap as it supports the weight of the camera, the one that comes with the camera is too thin and tends to eat into your neck after prolong usage, not to mention it gets dirty faster too.
The optech can also easily be detached by the clips and removed prior to keeping in dry cabinet to make the humidity level stable faster.