I am on my red P Plates in NSW am I restricted to conveting the engine?
Probably yes but 5k looks the same as what your 4k did so wouldn't be obvious.
Is the engine hard to convert regarding mounts, diffs, gearbox ect...?
All a bolt in conversion if you went for a 7k however I belive the sump requires modding, but if you keep it simple with a 5k its all bolt in.
What 5speed gearbox options do i have?
Your 4 speed will bolt up and then any 5 speed for a k series engine will bolt on.
and any other tips and ideas you could give me?
Just take your time and do everything properly then maybe think about suspension upgrades.