I chose to book in Sydney and found it not a problem at all.
Rental can be lower.
Perth, while less crowded than Sydney on the roads, has many one-way streets. I stayed at one of several hotels overlooking the Swan River. This allowed quick entry to the Albany Highway going south. Study them the night before your drive, and familiarise your passengers, so that all know the route and can help look out for landmarks and road signs. Do not shy away from motels for accommodation out of town, as they have facilities similar to hotels.