The headlights you can clean by just doing a cut and polish job on them, quite a few older skylines have this issue and they polish up real nice
10psi is perfectly safe to run on, I wouldn't go past that for daily driving really because you won't feel any real difference. If you must up the boost, just don't go past about 13-14, like I said tho, I wouldn't bother.
As for mods, upgrade fuel pump, injectors, buy a Z32 AFM and an aftermarket ECU and get it tuned