Hey there, i am not in Sydney and cannot help you out with a mechanic contact but what i can share with you is my journey into motorcycling. I too got a 74 550 about a year and half ago and too was interested in doing different things with it. I was new to motorcycling and keen to learn. I knew my first step was making sure the engine, gearbox and major running components were sound.
I was going to do the same thing and hand it too a mechanic. I am glad i didnt. I sat with it and started reading. Doing research, combing this site. You name it, some has done it. The amount of info on this site is invaluable. I was lucky to become friends with someone local to me who is basically an expert in what bike i have. He has helped me so much.
You can probably see where this is heading. My advise is to see what you can do as much as possible. Do your research. In the end you will come up trumps and have the satisfaction of knowing you done it where possible. Then farm out the bits you cannot do. Dont take it to Dues. If you work out exactly what custom stuff you want, you can easily save on taking it to someone specialised in exactly that item. Heaps of fabricators out that that will do anything you want if you clearly explain what you want.
Cheers man, have fun tinkering with your ride.