First of all, welcome aboard! Some thoughts, having just completed a 1600-mile tour through the Appalachians this past week:
1) depending on your noise tolerance, a taller / wider windscreen can be helpful. There are several good ones available, all of which have fans on this forum. I went with the CalSci, which worked very well for me (5'11", 32" inseam). The only issue I had was one day with very stiff crosswinds, but any taller shield would have been the same.
2) the standard sprockets are 15T front, 46T rear. Changing the front to a 16T, or the rear to a 44T or 43T (which is what I did), makes the throttle response much smoother, at the cost of losing a small (to me anyway) amount of acceleration.
3) bags / cases: whatever your budget allows for. I don't think there's such a thing as too much storage; that being said, there's an art to discovering what's truly necessary for a trip, and what's merely convenient.
4) GPS: I never really looked at the SW-Motech mount, but I did get a RAM mount kit from Aerostich which puts my GPS (an ancient Mio C320) at a perfect viewing location - easy to glance over to, and not blocking the main instrument panel. You'll probably want to put on a power outlet as well; Eastern Beaver is a good source for that.
5) seat: ah, the seat. Probably the worst stock feature of the V. There are a few good recent threads here on a DIY fix; if your budget allows, there are some aftermarket seats available; opinions vary.
The main thing, though, is that the Versys - however you set it up to suit your personal preferences - is an exceptionally capable touring bike. You've already made the most important purchase; enjoy!