Welcome to the club Andy.
The stero use speaker level outputs not low level
You can buy a facia plate from the US for the IS300 which will fit, if you do a search on the site I'm sure it will turn up or somebody will point you in the right direction.
The TC will come on by default every time you start up.
You can have a piggy back ECU fitted but it is expensive and the gains are slight I'm afraid, best bet is to fit an exhaust to start with and then look around. The quickest way to more power is to supercharge it.
Your best bet would be to come to the Gaydon meet this Sunday, plenty of cars to look at and people to get ideas & info from.
Where abouts are you from?