hey, welcome from VIC !
The most desireable sqbody will be the one with the least rust !
Doesen't matter what year, as so much is interchangeable, so you can build whatever you need once you have a decent rig.
I prefer the older ones ( look better ), and also have less smog stuff and computers etc., however they are so rare now over here that it's unlikely that you're roady will even know what to look for, let alone what was OEM, so it doesen't make much difference.
As you are in Sydney, assume size will be an issue ( we prefer crew-cabs, but too big for city driving ). Duallys are W--I--D--E, so be sure you need one before searching. Only advantage is stability, a single rear wheel will have more than enough towing capacity.
Likelihood of you finding a nice single cab dually here is very low. I have seen only 2- 3 in the last 15yrs. You are however very likely to find a C20 single cab with a 14bolt ( 8 lug ) rear axle and disc brake front. That was the standard spec of the Aus imports back in the day, and more than capable of towing duties.
Finally, stay away from low production RHD conversions - they are generally crap even if they carry a manufacturers plate. We are re-building my son's OBD right now for its roady - lower rad hose pinched between power steering hard lines, hood catch flipped over ( back to front ) so doesen't work, coreflute board puttied into the dash air deflector box, wiring disaster etc. etc.
Stay with factory LHD if you can live with it ( especially if a "loaded" truck - PS / PB/ PW / AC/ etc. ), otherwise i'd go with a "holden" import C20. You could even modify to a dooly rear if you needed to later on...
Good luck - post pics !!..
Especially in dark blue! 
And welcome from Wisco!
