Seeing as you just bought it I suggest that you have it fully serviced before doing anything else to it, especially if you don't know it's service history.
Replace....
# Engine, gearbox and diff oils
# Fuel, air and oil filters....Cabin filter too if it has one
# Brake fluid....Check the condition of brake pads and rotors while you're at it
# Clutch fluid....If it's a manual
# Coolant
# Clean the throttle body and MAF sensor
# Plugs and leads....Not absolutely necessary....will depend on service history
I do this whenever I buy a new vehicle regardless of it's service history
Some people say it's overkill, but I bought the car not them, plus I want to try and look after it as best as I can.
The main reason is to make sure that all those filters, oils and fluids are fresh and clean....plus I now know exactly when those items will be due to be replaced again
Only after I've done all that will I start to consider any future modifications
As for modifications......
There is literally hours and hours of reading available in these sections of the forum to see what other people have done to their cars
https://forums.justcommodores.com.au/threads/ls1-guide-to-power-stage-i.24382/
https://forums.justcommodores.com.au/forums/vy-vz-wk-wl-v2.106/
https://forums.justcommodores.com.au/forums/lsx-development-and-modification.52/?order=post_date