Its all about supply and demand. There are still large volumes of 240z/260z in the US and in japan so imsurprised to hear such high prices. special ones (432-r) will always be worth big $.
I see s30's as a 'steady' classic - not like a 60s/70 aussie muscle car is to australia or american muscle is to america. long term - i think those will outperform japanese classics.
In terms of modifications i think as long as you do what the japanese you will not hinder value. E.g watanabes, R200LSD, triples, even L28ET isnt a bad thing. In fact RB zeds are respected and accepted in Japan. Japanese car culture is about modifyng so as long as its tasteful and done properly then you will attain buyers rather than scare off. All in all though body condition is the most critical element.
High level, in australia:
A big part of the problem though is more people culture, space, housing prices etc. I think whilst there will be less zeds, there will be a lot less buyers of zeds too.
Having the space for such a large toy is hard these days. A lot of people would sooner buy a modern sports and drive it to work (2 birds one stone). eg SS/wrx, whatever you want.
I seems the main customers for s30s is the japanese.
I see expensive parts more of a hinderance to the future of s30s. Long term i think aussie muscle will perform better price wise (purel in the aussie market);