When you say spare parts, really what are you referring to specifically? Or you mean in general for monthly servicing, etc?
Obviously certain items are more readily available than others and the thing about Nissan and I'm sure other manufacturers as well is the common practice of using parts from different models (ie: an S13 using a GTR stock fuel pump, etc) as a replacement and usually fit without too much work.
That's before taking into consideration the product offerings by the after market tuners. If you had a stock GTR and wanted to upgrade, the amount of parts available from Jap/Aus/NZ/UK/US tuners alone will drive you crazy. It's that extensive. From a simple fuel pump to suspension to engine internals to a common gear knob to carbon bonnets - what you want is probably available.
But don't overlook the $$$ aspect of owning a GTR which can be a pretty tidy sum as it ages.
As for your second question, I don't plan on selling my car anytime soon. I dream of owning an R32GTR but I realize it's not the smartest thing to do. The R35 is slowly growing on me as time passes and once I persuade DC to sell me one cheap cheap, then who knows. But I see myself owning GTRs in the future, if finances permit more than one.