I've never driven a Skyline but I do know they are AWD. This will be the first difference you'll notice. I love RWD :teeth:
Also, not sure how much your GTT pushes, but I'm guessing it's more than the Standard Roadster... The Z is a modern car though, and tbh I can't fault them! A few niggles, but relatively minor and documented (here on this website, FAQ and Technical forums).
With regards to the modding scene, there's quite a lot of choice for both Z and ZR only mani difference would be for the rear spoiler due to the difference in shape of the ZR boot. Apart from that, quite a lot of goodies to mod your Z with!
So you the organiser of the speedball rally? sounds like good fun, and quite acessible price too, with leaving the choice of hotels to the participants! sounds like a great few days!