I'm making a list of things I need to turbo my sr20de. So far on my list are - sr20de manifold that fits a T25 turbo, an HKS T25 turbo, bov, downpipe. What else is needed? I now things like the wastegate come on my turbo (or do they?), but what about an intercooler - is it needed? And a boost guage is needed but where does it go? Your input would be helpful.

This is the car:
1992 Nissan Sentra SE-R sr20de motor w/AC

rather than list everything, I'll link you to Trav's site with a list of everything his kits include. If you're building your own kit you will need almost everything, if not everything, off of this list.

http://www.forcedinductionracing.com/B15SR20turbokits.htm

How about for a 99 sentra GXE Limited Editition (yes I am changing from Auto to manual) Do u have any specific links on tuning it?

generally you will need everything that's on that site. The only things that will be different are the ECU and maybe some of the piping. For the ECU you can just get a JWT b14 one, and the piping you would probably want to talk to Hotshot. I'm not sure if they have anything premade but they could fab something to your needs.

One more question , is it worth it to add NOS to it or would it just be harmfull?