Best bang for buck is quality speakers in the factory locations.

I'll give you an example in my FG, but the Commodore isn't that much different - just the sizes & tweeter locations (I also have a VE Calais V).

Stage 1 cost me just over $400 and consisted of:
- Polk db 6.5" splits in the front doors & A-pillars to replace the factory gear.
- Polk db 6.5" 2-ways in the rear doors
- Sound deadening applied in each door (not completely covering - just enough to damp it.

The Polk dbs have an excellent sensitivity rating - meaning more volume per watt - far better than the factory gear.
I went for probably 2 years like this, before wanting a bit more out of the factory head unit.

Stage 2 cost me around $550. I upgraded to a Pioneer free-air sub in the factory location, and a Pioneer 4-channel amp, mounted under the front seat.
It has a remote level control for the sub, and I run it off the bridged rear channels of the amp.
I routed the front speakers through the amp's front channel too. Made a huge improvement.

The only things I could do now (for reasonable cost) would be:
1. A boxed sub off a bigger amp
2. A separate aftermarket head unit to run the audio only.

You can spend big $$ on digital signal processors and other stuff, but where do you draw the line for value?

The reason I noted using an aftermarket head unit, is the factory one is a bit noisy, and over-processes the signal way too much for my liking - both Holden & Ford are like this.
Things like over-use of dynamic compression to go loud without clipping, speed-linked volume that impacts the dynamic range negatively, reduced bass output when the fader is centred - as the inbuilt amp can't produce enough power - and many other things that are purely cost-driven during design & manufacture.

But the problem with the VE onwards (and the BA onwards) is the HVAC system and other vehicle data is accessed via the screen and audio system, so you still need to retain it, or use an expensive aftermarket solution.

In your VF I would start with decent speakers in the doors & tweeter locations first, and see how you like it. Then look to an amp for those speakers, and a sub of some sort.