Would this local camera store allow you to put your own memory cards into each camera and shoot a few pictures so that you could take the cards home and view the images on you own computer?
I do this all the time. Often, I just take my body and try out different lenses on my memory card. Most dealers won't mind at all.
To take full advantage of the FX format you're going to need some of Nikon FX lens, for example the holy trinity, these tend to be a lot more expensive than their DX prosumer counterpart.
I wouldn't agree with that at all. Few shooters need all three f/2.8 zooms. Especially hobbyists. Someone with a new D800 would have loads of fun with just a 24mm f/2.8 and an 85mm f/1.8, and would have all of the benefits of shooting with a larger sensor. If budget isn't a huge issue, a new FX body would be preferred by most.
The decision to go FX is tough, as I know many others here have suffered through the same spot I'm in... Ideally there would be some kind of photography group in my area perhaps I could get involved with to talk to and see first hand some of these differences... sigh...
Why tough? You already said that budget isn't your main concern. If you choose an FX body, I think you'll be happy you did. And, since it's a long-term investment, your cadre of FX glass will grow over the years. I own only one of the holy trinity lenses (70-200mm f/2.8 VR I), and I love it, but I also have no want or need for the other two, since I prefer even faster, fixed focal-length lenses.