Well, most services as such are Australia wide so being in SA shouldn't matter (hell I get the same services as most of Australia and I'm in the middle of the outback).
As far as broadband is concerned it's best to shop around to find plans that will suit you, everyones requirements are a little different.
I personally use
iiNet. They have great plans, very stable service and are gradually buying out just about every other ISP out there (which is how I started using them they aquired the ISP I was originally using).
I've been told
Internode are quite good also.
Just whatever you do, don't signup for Telstra Bigpond. Usually they require entering into a 2 year or so contract and by far have the most expensive prices.
As for mobile phones it's a little different.
Telstra, like BT is the major telcommunications provider in Australia. As such they tend to be the most reliable providing the best coverage around Australia. Depending on what plan you choose if going for a contract they can be OK price wise. Alternatively you could go for a prepaid mobile. I am using them for my mobile service currently.
Optus were the first major competition to Telstra. They have some decent prices but aren't always the most reliable service carrier. Technical support is a bit crap with them. They don't have anywhere near as good coverage as Telstra, not really an issue in city areas but if you decide to go in some country areas or generally non populated areas you may find yourself without a service available.
Vodafone you should already be familier with. They haven't been in service as long as the other two but mostly they seem decent. A lot of people who were previously with Optus have switched to Vodafone as they tend to be a bit more reliable and have much better tech support. I don't know what their service coverage is like but I imagine it being a bit like Optus. As these third parties haven't been around as long they have yet to put the required infrastructure in place to be able to give as extensive coverage as Telstra.
That should be enough to get you started, like I said though it's a matter of shopping around to find whatever service & pricing will suit your needs.