Just about the whole coast of Australia has nothing but beautiful beaches. If you go to the famous ones, like Bondi, you will barely find room to put your towel down, especially in summer. As you travel along the coast, you will find many beaches, where you will have heaps of room to roam. We DO have some nasties in our oceans and in amongst rock pools, so never pick up anything and place it on your hand. A harmless looking shell, could have a stinging creature in it, and we have a very small brightly coloured octopus, the blue ringed octopus that is deadly, one sting and you have a 99% chance of dying. We used to find them as kids along the rocks near Port Lincoln's Curtin Point caravan park. That was before the newspapers reported on a man that died after he put one on his hand to look at it and got bitten. Hopefully you will be too busy taking in the wonderful scenery to be too engrossed in the smaller life forms that live on the beaches.
As for our sun, as many have already warned, it is deadly here. 15 minutes in the sun and you WILL be burnt, even if the weather is cloudy. It is absolutely essential that you equip yourself with a decent hat, sunglasses and good quality suncream, SPF of 30+ or more, and a decent shirt. You can find some lightweight longsleeved cotton fishing shirts fairly cheap that will keep you cool but stop you getting fried.
You can find cheap accomodation in many of the smaller places along the way, but if it is a touristy place expect to pay a decent rate, anywhere from $90 - $130 per couple per night. Caravan parks are about the same for a self contained cabin, a caravan may be a bit cheaper but you may have to pay for linen if you dont have your own, which you probably wont. If you drive inland to a small town just off the coast you may be able to get a room at one of the pubs for a reasonable price, but it may have a shared bathroom rather than an ensuite. We have payed between $50 and $70 per night per couple at some country pubs and the beauty of this is that many of them serve meals for a decent price and you can have a drink or two and then wander upstairs to sleep, you wont have to drive home, and some of these old pubs are unique. They may be a bit dated, but they have character and some of the staircases inside are beautiful.
I am sure, with the help of the people on this forum, you will be able to have a decent and reasonably cheap holiday and you will get to see things that the other tourists do not get to see, with less people there to block the view. You dont think that we want tourists to see ALL the good stuff do you??