Get in contact with an engineer before you start spending money. Outline what you would like to do and he will fill you in on what you can get away with.
Unfortunately a Cobra is considered to be an "Individually Constructed Vehicle" rather than a modified vehicle based on an old chassis or body etc. This means it must meet the Australilian design rules the same as any new car. Not too much of a problem except this includes emissions which rules out most of the desirable and reasonably priced power plants. You will probably be stuck with running a 5.4 Ford Quad cam if you want to stick with the blue oval or the new L76 or LS2 GM motors.
Picking up a wrecked BA is probably a good option as you will get the engine, trans, electrics and lots of other little components you don't think about till later i.e belts and pulleys and fuel fittings and other stuff.
No one seems to be building a kit that uses donor parts all from one vehicle. Most use mixtures of parts from Jag, Ford, Holden etc.
I hear a rumor that Classic Revival may be changing their chassis design to accept BA falcon suspension components. It might pay to chat with Ian to see what he has planned. His new development may work in with your time frame.
The issue with side pipes is that they exit towards the foot path. The drivers side pipe is legal but the passenger side is not. It's pretty much irelevant as from experience its nearly impossible to get side pipes down to the legal noise level and still have the engine run. Build an under body system and then change after rego if you want to run side pipes (for the race track only of course
).
Tyre diameter depends pretty much on your personal choice. People running 17" rims are generally running 275/40/17 or 315/35/17 both which have a diameter up round 645mm. The guys running 15's will often run 295/50/15 rears which are up around 680mm diameter. The 15's certainly fill the guards up nicely.
Most Cobras seem to weigh between 1050 and 1250 KG depending on drive train choice etc. There are some heavier bodies and chassis available and some light ones. There are some super light cars that may get under the 1000KG mark but they'll be pretty well stripped down with no creature comforts like carpet or windscreen etc.
The only stupid quastions are the ones that don't get asked so fire away. These threads often result in some good discussion.
Cheers