Hi Mike.
Welcome to the world of cnc.
Im fairly new myself ( to cnc )and there is a lot to learn. It can be daunting but when you get the hang of it and watching your machine cutting parts its all worth while.
Cant help you with the seig questions as i built my own cnc from scratch.
Down in melb. you have Homman Designs and Ocean controls who can help on the electronics side of things.
I know most people have a budget and often look to the cheap ebay electronics but i believe in spending a bit more and get some decent quality stuff like gecko from Homman or Leadshine from ocean controls ( where i got my steppers and controllers from ).
As far as software for cnc you need 3 things, cad, cam and machine control.
Download fusion 360, its cad and cam in 1 program and free for hobby users.
You draw ( design ) what you want in cad then use cam to oranganise how your parts will be cut. Then send it to your machine control which move your mill to cut the parts.
So 3 lots of learning you have.
As far as machine control you have mentioned mach 3 but there are plenty others.
Look at uccnc, its what I'm using. I started with a mach 3 demo and was going to purchase a license. But Uccnc is newer and works out cheaper and i am relly happy with it.
Anyway hope i haven't put you off.
There is a lot to learn but its good fun and not that hard when you get going especially if you are mechanically minded which sounds like you are.
Ps i used to live in Vic, now in Qld.
You guys have still got tight restrictions going on down there. I don't miss vic 1 bit.
Cheers.
Barry.