of course, ventilation fan on the roof will help cool temperature down, but I don not think it will take the moisture in the roof because the house alread can breath through the roof tiles. personally, I do not like to put a ventilation fan on roof. Synday weather acturally is bit dry than Melbourne, so I do not think your roof tiles is demaged by moisture. If you only want to cool down your house, it is better to put isolation material in the roof space.
so now I am start to think about installing a ventilation fan on top of the roof, to take out the moisture within the cavity of the roof, what do you think? and the fan will help to cool down the house during the hot summer too.
CERAMIC TILE IS BETTER THAN CONCRETE TILE ANYWEY,...THEY ARE VERY POPULAR AND TRANDITIONAL IN ALL STATES IN AUSTRALIAN. YOU ONLY NEED TO REPLACE BROKEN ONES AND NO NEED TO CHANGE WHOLE ROOF.