Carla,
How long is a piece of string, indeed! As a rough guide, the stereo is only a few hours work, as the dash and electricals of this model are so simple. Fitting disc brakes is a few hours work for a mechanic, providing they have all the parts required at their fingertips - see the article at
http://www.fefcholden.org.au/techinfo/tech002.html for more info on this conversion.
A new motor is probably a day's work for a competent mechanic, again assuming all the parts are at hand. However, if you are talking about a different size engine (as I presume you must be based on your reference to a/con), then it is anybody's guess. It might be a relatively simple upgrade to a red-motor (later Holden six), or a V8 conversion that would cost you thousands. I don't know what you have in mind, and you would have to speak to an engineer in Queensland about what's legal before embarking on this anyway. Ditto for the air-con. If you're looking to fit a late-model V6 or something, then you may be able to get the a/con with the motor, so at least it's all complete and just needs to be shoe-horned under the bonnet. But anything else and your talking about a serious labour of love for someone who wants to do this for the hell of it - read very expensive.
I hope that helps. Make a start with a conversation with an automotive engineer before laying out any cash, and perhaps try the FE-FC Holden Car Club of Queensland for references on mechanics. They can be reached at
committee@qld.fefcholden.org.au, or on 07 3886 7361.
cheers Richard