hey wes, have you got a 4wd ?
There's lots and lots of beach for down-winders !
North Straddie -
It might still be blowing slightly west in august, but you should be able to find a few spots.
Be careful of amity point, very sharky !
Otherwise main beach around to cylinder is awesome for surfing.
South straddie is less developed, i'd stick to north straddie, if your taking the family. More accommodation, a few restaurants, bakery, servo and so on. Its still quite small, but there are great houses to rent.
The first pic, is from a house I stayed at in February, overlooking cylinder beach - i can give you the details if you want.

You would be much better off heading to Fiji in August for all of your requirements I reckon - and still probably get a lot more value for your money for everything.

Stradbroke Islands are great Summer spots - very average winter spots.

If you're heading to Couran Cove Resort I would leave the kiting gear behind unfortunately.