Hi cryseee and welcome aboard the good ship Salehoo : )
Most of your questions really come down to your personal preference really. Should you only sell in Australia or sell into the US market as well? I could only suggest that you give that one careful consideration before deciding really.
In regards to Drop Ship suppliers, the chances are you would be dealing with international businesses there, so you really want to make sure you are dealing with excellent customer service, because when you have a problem, you will be relying on them sorting it out for you, so you need to have the confidence in the business to know they are going to do the right thing by you and your customers.
In regards to the cost of sourcing from multiple suppliers. Of course each will have their own pricing, their own shipping costs and shipping times, but that's up to you to keep your customers fully informed as to the conditions of trade on a per item basis. So they will know the cost to purchase, the cost to ship and the shipping time.
If you ensure you cover each item informatively, there shouldn't be an issue in dealing with as as many suppliers as you are comfortable with.
Cheers