First things first: this comprehensive Point Hacks guide to booking around-the-world tickets using Qantas points will answer most, if not all, of your questions. Read the whole thing, but pay special attention to the section “How to Research and Book a Qantas Oneworld Classic Flight Reward”.
Also, believe me, it’ll help if you sponge up anything you can find on Point Hacks about using Qantas points, or Oneworld reward flights. Spending a couple of hours reading and digesting as much as you can now, will potentially save you a lot of heartache later.
A few more comments:
You can’t convert Qantas points to Qmiles. You will be booking your tickets using Qantas points. You can, of course, use your Qantas points to book a Qatar flight or two as part of your RTW trip.
To answer your specific question about the cost: it’ll cost you 318,000 Qantas points per person, for a RTW trip in Business class.
You will find it extremely difficult to find 4 Business class reward seats on each trip for a RTW flight. Even 3 will be a huge, time-consuming, frustrating, possibly-futile struggle. Finding 2 Business reward seats per flight is much more realistic. So yes, your strategy of being willing to pay for one RTW ticket using money is a good one; I’d even go further and suggest that you look into paying for 2.
Finally, don’t forget that the American Airlines and British Airways websites can be very useful backups in searching for reward flights that you mightn’t necessarily find on Qantas.
All the best and let us know how you go!