i think maintenance and mods get mixed up sometimes...
keeping a stock R33 GTR maintained wouldn't be so bad, a little bit thirsty on the fuel, 17" tyres aren't too bad, heavy duty single plate clutch would be fine... as ben said lots of parts are readily avialable...
its more when you start moding, and have to do maintence as part of that
ie my car is a bit lower than normal, so i have a bit too much negative camber, so i need to put a camber kit on etc...
my wheels are 18" so tyres are that bit more expensive...
its running a bit more boost so its a little bit thirstier.. etc....
but no matter how good you think the car is, always budget a bit for things to go wrong, its a 8-9 year old performance vehicle, people don't buy GTR's to drive them like camerys, they buy to drive them how they are built to be driven....