Yes get air con. As braaay says almost everyone in Perth has air con or fans or both. I have both and I live 1 street back from beach in a dip. If you can afford reverse cycle air conditioning that would be the better way to go. Evaporative just does not work after about 3 days of intense heat. If the house you are purchasing does have air con just make sure it is in good working order before you buy.
Kallaroo is a beachside suburb, so there will be nice sea breezes coming through in the afternoon. It would depend on exactly where your house is though, perhaps it will be in a dip or up a bit higher and that will affect how much cooling effect you get. You might last a bit longer than someone inland before turning it on, but it would be worth having. In Perth pretty much everyone has an air conditioner or fans to cope with the summer heat. So I would get one and in any case f you buy a house there it will probably already have one or more installed.