Hey,
Have a look through last-years posts on this topic, cause heaps of euro backpackers are always asking these same questions - you'll find a lot of your queries answered in those old posts.
In summary, I'd suggest bring everything except the 15m kite. Winds over here are generally pretty strong.
Word of advice - winter winds in Perth are notoriously gusty and dead onshore, which makes kiting in Perth in winter a bit tricky.
We very unfortunately had a guy killed 2 years ago, cause he went out on a gusty westerly day, with a 9m kite, and got hit by +40 knot gusts and slammed on to the shore pretty bad (well, really bad acutally). Lots of other injuries occur in winter as well. So, if you're here in winter, be careful, take it easy at first, get to know some locals, learn the safe beaches and how to read the storm-fronts / squalls, and what to do when the fronts / squalls come through (i.e. keep an eye on the horizon and when the dark clouds / rain start coming, drop your kite cause it's gonna get hectic real soon!).
Up north throughout winter, say from Geraldton to Exmouth, you'll find epic kiting everywhere. Because it's remote it's all super uncrowded which is amazing (except Gnarloo which has a pretty heavy kiting / windsurfing scene). The flip-side of remoteness is that any screw-ups mean you're on your own a long way from hospitals etc, so kite with a buddy up there.
Lodging on the way up north - just camp on tracks off the main road if you wanna save cash. Lots of little camp grounds in the small country towns. Rent / buy a cheap van, find a hot backpacker chick from the OB backpackers in Cott and get on the road and go explore!

Cheers.