if the car comes with rego and it has more than 3 months you have 3 months until you have to get it over the pits. but look into it as it may have recently changed.
as far as mods go you shouldnt have any trouble as down here we are nowhere near as stricked as the mainland. my car passed with visual things like pod filter, bigger turbo, lowered, intercooler ect. and sound wise 3inch xforce exhaust system (pretty loud), and atmo bov (pretty loud) ect. mainly just make sure your tyres are good, there are no fluids leaking and your seat belts are in top condition.
in other words you should be fine as far as mods go.
its always a good idea to check the vehicles registration status, even if you are told its registered and it has a clear title. get registration number, VIN and engine number and check it out before making any advances.
have fun!
4drftn