I love Nuno's tone too, especially on the 1st, 2nd, 3rd and 4th extreme albums... (hang on a minute
Personally I think Nuno's tone is readily available from the N4 just as much as Vai's tone is easily available from the JEM7VWH. It's in there and it's easier to get something that sounds like the guy than it is to get a tone that sounds nothing like the guy.
The biggest difference for me is firstly the 22 frets, which is ok cos I'm sort of used to that with all the JS
guitars around here. But if you're not used to the switch between 22 and 24 that's something to watch out for.
The other thing is the funny "Stephen's extended cutaway" neck joint which means that you kind of have to play with your thumb behind the neck much past the 15th fret or so.
Oh and the weirdest think for me at least is the switch isn't where you expect it after playing an RG.
if you find a cheap one second hand or even dare I say it... on ebay, treat yourself, I'm sure you can move it along for what you paid for it and you'll have had loads of fun along the way
Particular ones to look out for are the padouk (paduak?)ones which have a lovely raw mahogany sort of sounds. Or the ultra crisp swamp ash ones (later models). Or you could go for the original Alder bodied ones. Oh and they also did some limited edition mahogany/quilted maple ones too, in both black and cherry sunbursts.