don't worry about never been boosted.
they've all been boosted.
stock air box is a good item that's why there's no need to change it.
GT-SS are good items too.
If your mechanic says it's good, and you good compression test results and no major problems throughout the car, AND he accepts your offer, it sounds like a good buy.
non performance related stuff you REALLY should make sure you get.
1.) Comprehensive insurance - if you can't afford this, then you can't afford the car.
2.) Good immobiliser (2 or 3 point is the standard that most insurance companies REQUIRE you to have)
3.) About $1000 spare as "just incase" money and for the first service you do where you should change all fluids (engine, radiator, diff, gearbox), all filters (air, oil, fuel), all belts (fan, timing, ac etc).
If there is no receipt for the timing belt change and the car has over 100,000kms, just do it. Unless ofcourse you pull the cover off and see a Gates timing belt that looks newish..
You'd want to change the water pump and idler when doing the timing belt too.
good luck on your new purchase.