Read the HSV/Monaro forum mentioned above.
Brakes aren't £2k, that's the install cost for APs. Brake discs range from a couple of hundred for a full set on the 2004 CV8 (pretty crappy) to ~£650-800 for a full set of DBAs to fit the VXR. Performance friction pads are £200.
Not much to go wrong with them, just be aware of gearbox problems - a new 'box is ~£2800. The clutch fluid tends to get filthy and can cause you to lose the ability to declutch. Simple fix, but potentially annoying if a remote bleed line hasn't been fitted.
I've got a 2004 CV8 with some mods - the difference between the VXR and CV8 is styling and a few tweaks (read up on the HSV forum). Many CV8s are modded to VXR level or higher. Note the differences between the 2004 models and 2005/2006 - the fuel tank got moved into the boot in later models and the exhaust changed to exit on both sides.
Consumables and servicing are pretty cheap really, e.g. the tyres on mine are 235/40/18. CV8 bumpers and bits are readily available from breakers, but equivalent VXR bits can be much more expensive.
Any garage can do most bits on these cars, but Monkfish and Walkinshaw are specialists (as said, avoid the Vauxhall network).