Generally 1/10th scale is a goer.
You can buy kits starting from about $500 ready to race. The more full on you want it to become, the more expensive. Kit Race Cars are about $1k without any running gear, so $500 with everything is a pretty good starting point.
RS4 is made by HPI (yankee brand). Have a look around at a hobby store ( I use Hobby Habit , Main North Road, Blair Athol (near sefton plaza)) You can buy a GTR shell from either Tamiya brand of HPI, they're about $50 each unpainted.
Maintenance is low, i'll let dean tell ya what there's 2 do with nitro since he's raced at littlehampton.
www.modelflight.com.au for a bit of information on cars in SA, it's limited information but its a start.
Hope it helps
Tim