many of these can I use/how far can I go without having to get the vehicle inspected/certified/plated to keep it street legal? (Queensland, Australia)

Basic modifications do no require formal approval from an Approved Person or the Department of Transport and Main Roads. But any person performing a basic modification must ensure they meet all the requirements listed in the NCOP, and any applicable requirements of the Australian Design Rules and the vehicle standards.
• Fitting a replacement new, used or reconditioned engine
• Fitting equivalent engines with an increase of up to 20% of original power.