ORAC, fobs, thank you very much, you've made my day.
With your info at hand I rang the Oz D-Link office to discuss my proposed set-up (I found out previously that there are issues with my DSL 200 modem that need to be considered). It is clear now that the 520+ is the way to go for me, and in being able to make that clear decision I've saved more than OZ$100.00 over yesterday's draft proposal that might have had problems.
It pays to shop around for both prices
and information. My project cost has come down from $580 for what I was first told that I would need to $214 for this much simpler and perfectly adequate system.
Fobs, I'm attending the door after the horse has bolted, but did you consider a high-gain indoor antenna for your 520+ to address signal penetration before outlaying your hard-earned on the 900+? D-Link make a number of them suitable for the 520+
Edit:
The guy who suggested the antenna as a budget way of improving the range/penetration of the 520+ signal said it cost about AUD$70.00. I checked with an on-line discount retailer. He was wrong - that's just the cable. The antenna is about $200.00, for a total of about $270.00 which makes it a not very cost effective option after all.
Thinks ... The little dipole on the 900+ might not improve your coverage, fobs, the transmitter power on both the 900+ and the 520+ are the same, as are the antennae and their gain. I hope you prove me wrong.
BTW, fobs, what are the walls of your house made of, to cause rapid signal loss? (Or how big is your house? 
) Mine is 2 storey, timber frame and upper floor, plasterboard interior walls and ceilings, brick exterior. Shouldn't present a problem, one hopes.
Cheers
AA