I was leaving the answers here to some NSW Ford guys but it looks liek they are all quite.
1 = Where I am living I had to apply with the local government for an "approval in principal" to build my car, on this application I had to list all the drivetrain, brakes = type and size, Wheel size, just about the complete car in writing.
If this is the same in NSW then i would want that certificate as proof of build start date.
If the car was started before 1 January 2005 then as Tenrocca says you must compy with emmision reg 79/03 in which case your 5 litre ford with efi or lpg is ok (on a side note to that I do know a guy in SA who has used a 5 litre engine from a truck in japan (I dont know what truck) that was built to 79/03 regs and used a carbie, he has succesfully registered)
2 = This is probably more personal choice than anything, If you want irs then go with the jag, if you are going to make a shyt load of power then go live axle with 9 inch.
The drb torana kits are the most common in Australia as I think they are the cheapest to assemble (not a bad thing)
Also if you are thinking about some fun on the track then the jag (irs) is probably better.
3 = I guess I answered this in number 1.
But if starting today and you wanted V8 then either LS1/LS2, Ford o/head cam (current) or and I could be wrong here Lexus V8.
4 = I dont know what kit he sells so I cant answer this one.
5 = You need to decide what you want out of the car.
Depending on how far these cars have been completed then check the quality of the parts used, Check how well it has been put together.
Some bodys follow the manufacturers original lines better than others, some have the body bonded to the chassis. All these are personal preferances.
6 = My engineer is alittle to far away to help you
Good luck in what ever you decide, If you get some pics and post them in here before purchase I am sure you will get more feedback.