It's a pretty basic process depending on the speakers you want to install. Have you got a budget in mind??? Do you have an idea on the speakers you want??? Do some reading here
http://www.fhrxstudios.com.au/ and you will get all the info you need on how to properly install speakers. One thing I will say is steer clear of Autobarn, Strathfield Car Radio, JB Hifi etc... They are all hacks.
Can't comment on the installation of a HU in a GenIII but it couldn't be that difficult. You may need to buy a fascia kit to install an aftermarket HU, and also look into buying a wiring harness from any car audio shop. Aerpro make really good wiring harnesses.