Given the price of upgrades from Economy to Business class, I would suggest looking for outright award bookings ( 128k vs 92k iirc )

If looking to pay premium economy fares, I would also suggest looking at how much it would cost for business class outright on another carrier; Given QF's fares there may be little price difference and save 184,000 miles and no gamble

e.g. looking in October, Qantas's price ( from ITA) is showing at $5650 for Premium Economy ( vs $3285 on JL, $3490 on SQ and $3670 on BA ) ; compare that with the lowest business class fares on offer at $4833 on Air China , $6017 on Finnair (with AY, the SYD-SIN is on Qantas anyway ) and $6136 on TG

The purchase of PE on Qantas and then the lottery to try and upgrade for at $5650 seems much worse than just purchasing the Air Chine ticket upfront

There is no real secret to getting an upgrade to clear - off peak times of year are likely to have less competition for seats; holding platinum status makes your chances of success better

I wouldn't purchase PE just to improve chance of getting an upgrade since other business classes could be cheaper ; buy an econony ticket and hope for an upgrade to PE or business or just buy business class