I haven't got the Tamiya kit yet (I have 2 on order from O/S) however, I have built a couple of CAC mustangs. 2x Dragon 1/32 kits & 1x Airfix 1/24.
Decals can be sourced via ebay search novascale 1/32 p-51 & you will find them, I believe there is also another decal manufacturer currently preparing a sheet, they also do spitfire stuff although I can't remember the company name.
With Aussie Mustangs you really need to check your references as there are many variations depending on place of manufacture time period Mustang model etc.
The Tamiya kit currently available will produce early NAA & CAC built RAAF Mustangs only, The Rocket locating holes & studs were present on most RAAF Mustangs. From what I've read, there will probably be a P-51k variant released in the future & this will represent the bulk of CAC built mustangs with rocket stubs, Aeroproducts propeller, battery in engine bay etc. The CAC built Mustangs also had a formed (moulded) plywood seat which was unique to Aussie built planes. Also depending on the aircraft, there were also diffeneces in access panels for the engine etc. Best bet is to check any references you can find, the best sources are...
Red Roo models book building the CAC Mustang bu Garry Byk, these guys also have accurate alternate resin propellers, radio gear & other mods for CAC planes (check their website).
Spitfire Mustang & Kittyhawk in Australian Service by Stewart Wilson
There is also the CAC Mustang bible Southern Cross Mustangs by David Muir
Hope this helps!