Mate, I'd recommend leaving it too the experts as they have insurance if they fu*k up, easy mistake could cost u a motor .. But to clean all the crap out properly you have to remove hoses, thermostat, header tank and flush everything including the little hose between the radiator and header tank, and heater core. Even then v6 holdens can be hard to get clean. The brown crap always seems to come back.. :b:
I can recommend Ultratune in Surrey Hills, Knox and Croydon as i know them. Also Technotune in Nth Ringwood are supposed to be good. From memory, don't quote me, full flush with anti-freeze coolant (ALWAYS in a V6) and a new thermostat (im guessing hasn't been done yet, highly recommended) should be about $120-150. Its been quite a while since i did one so i'm not 100% sure of costs. When you drop your car off get them to confirm exactly what they will do,what type of coolant etc. Don't be afraid to ask questions,
NOTE TO ALL WHILE ON THE SUBJECT:
NEVER TOP UP COOLANTS UNLESS YOU ARE USING EXACTLY THE SAME BRAND AND TYPE, MIXING BRANDS/TYPES CAN TURN YOUR COOLANT ACIDIC AND IT WILL EAT AWAY AT ALLOY ENGINE PARTS.......