welcome aboard, for places to get stuff done,
Mechanical - i have been using Fours N More for the past 5ish years, built, put in, tuned and everything else you could imagine they have done to the RB25 in my r31 drift car. 82602011 is their number and Brenton is who you want to speak to. Tell them Luke that drifts the R31 sent you and they should take care of you. They do alarms too.
As for audio, Jb's at modbury have a really good installer if he is still there, havnt worked there for about a year now so not sure if he still there, but did alot of custom work and basic installs too, they can sort you for the alarm too. If you head in there talk to Pep or Kahul in the car sound section and again tell them Luke that used to work in the games section sent you in and they should help you out.
Best of luck with it all, sounds like your gonna turn it into something nice.