Ok Pippa, I am probably going to get shot down in flames by people who know there geography better than I.
You can probably expect some tree cover until you get to Augusta, however I would not rely on it to protect you from the sun.
Weather? Try this site and find the towns along the ride: http://www.bom.gov.au/climate/averages/ ... ames.shtml
I would expect Albany is as cool as it will get and Perth about as warm as it will get.
How far will we climb? Still waiting for Main Roads Mike on this one, but WA is not a particulary mountainous or even hilly place.
What does it look like???
I am a West Australian and when I went to Zimbabwe in 1996, I was surprised how similar the Zimbabwe bush was to the bush around Perth (particularly as the veld grass has taken over). Of course there are no large wild animals. As you go south from Perth the trees get larger and it gets moderately cooler.
Here are some pictures I have found from along the way:
Manjimup:- http://www.southernforests.com.au/produ ... Attraction
Margaret River:- http://www.olivehillfarm.com.au/album/
Between Busselton and Perth on the ride there is still a lot of bush but also more develoment and farming, I could only find beach pictures.
Can I ask you is it worse to have your tyres freeze to the road at -40 or melt into the road at +40? (it won't be that hot).