Haha, great welcome and read guys.
Welcome to the forum Monkey.
WA specific I can't help with sorry.
Registration (Rego) is like MOT. you pay the government to be allowed to drive your car on the roads.
Yellowslip = Defect notice? (WA may have slips?) If your car is suspected to be unroadworthy for any reason you can be issued a defect which means your no longer allowed to drive on the roads until you have inspection proving repairs/mods removed. (If still rego'd you can drive to place of repair)
When buying you can do a REVS check or some sort like it. For a small fee you provide the engine number and rego number and they conduct a history search checking if stolen, been in accidents, has finance, etc.
I recently spent 3 years in the UK so I know what you mean about the 'general' lack of decent car enthusiasts. Most of the 'real' cars are just your top end expensive ones like AMG, Audi R8, etc.. as for 'normal people' cars they are very rare. You will find, as with most things here, Oz is pretty much half way between US and UK.