Legal mods will depend which state you are from.
For Example, in VIC, you can only have 1 intake mod.
So it's either air pod(s) or front mount intercooler.
Can't have both unless it is a GTR.
I think that also goes for NSW but if you have a pod(s) it needs to be boxed / shielded off.
I think QLD will be along the same lines.
Stick with in the legal mods, you will not need any engineers cert.
If I were you, I would keep the car stock, get all the road worthy / rego done before starting any modifications, just to save you all the headaches.
If you put adjustable suspension on that's fine, as long as it is at legal ride height.
Lowest you can have your car is 10cm (100mm) above the ground. Lowest point is the exhaust (I think).
If you put a high mount turbo on, it will stick out and cops will pick on that straight away OR get an Engineer's Certificate to say it is legal.
Not sure about QLD, but in VIC you can get an Engineer's Certificate as prof to say the modifications on your car are all LEGAL.
For example if you want to put on a carbon fibre bonnet (illegal in VIC) you need an engineers cert to say its OK to have.
Or if you got a Road Worthy Certificate, meaning a mechanic has checked over the entire car and has passed then that should be enough prof to show the registration office that your car is legal + depending on how strict ppl are at the registration office, some don't care or won't know what's stock and what's not and just pass you.
This may also apply in QLD, but nothing to worry about, if your car is stock.