I want to buy nwn to play on a specific server that hasnt migrated. So I think i need diamond. Can i just install the diamond bit when i buy enhanced? Any advice welcome

If the server hasn't migrated you need to install Diamond Edition. When you buy the Enhanced Edition you get also access to Diamond. These are separate titles in your games library, so there's no problem to install only the Diamond Edition.

thanks. So i can just buy enhanced and install the diamond bit... is the fact that i want to play on my mac be a problem? enhanced is for mac too i think, but maybe the diamond bit is different

Judging by this thread, it sounds like Diamond doesn't natively run on OSX and you'd need to use Wine or something like that. One user also warns that you should "avoid the upgrade to OSX 10.15 (Catalina), as that will disable 32bit app support" (I take it Diamond is a 32bit app).