Many/most were used hard and certainly not babied. Most saw a lot of use by news and wedding photographers. But an excellent camera. Greg Weber (
http://www.webercamera.com/) is the "go to" guy for repair in the US; don't know about Australia. I used one about 25 years ago for aerial photography.
90mm lens was standard. 180mm lenses relatively easy to find. 58mm and 60mm wide angles around but expensive (you also need the accessory finder that came with it). 135mm lens more difficult to find. Very sharp lenses.
Some models had backs that could be changed in mid roll and others not. If there is a small darkslide immediately behind the lens (when looking at the front of the camera), it's one of those models. Of course, the darkslide may be missing so look for the slot.
From my experience, 2 main things to watch out for:
Frames may overlap, especially toward the end of the roll. The push-pull advance, for all it's benefits, was prone to wear from overly enthusiastic use. This is a function of the interchangeable film back, not the camera body itself. I've heard mixed comments as to whether it's repairable.
Lenses use a Seikosha leaf shutter. Like any leaf shutter, it may "gum up" with lack of use and require a CLA. Parts, if needed, are no longer available and may require a "donor" shutter. These are, btw, the same shutter in the earlier chrome Mamiya TLR lenses and IIRC, parts are interchangeable.
I would also look at the Mamiya MF rangefinders (23 Standard, Super 23, Universal). Advantages include:
Ability to go 6x9
Availability of lenses with Seiko (as opposed to Seikosha) shutters, although earlier lenses did use the Seikosha shutter