QF preferred seats are only a bunch of seats closer to the front. The higher your status, the further to the front you can go. If you REALLY want the best seats out of what is presented in the seating diagram (after you pay for the ticket), you'll have to pay for them (eg aisle or exit rows). Preferred economy seats won't get you Premium Economy. At T-80 hours more unclaimed seats will/may open up. This way you can move further up in the cabin or you may get an aisle.

If you want cheap Business seats then you'd be better off looking further afield - eg the Chinese airlines, PR, FJ, and so on. It'll involve stopping over somewhere but you'll be paying less. Sometimes DL or UA might start heavy discounting and QF matches a few days later - for a few days only. Good luck!