Any home set up - even a basic or second hand one is going to cost you money, so it is as well to get at least some knowledge, to make sure that this isn't just a passing interest!. Even if you just do one gig with an established D.J then it will give you some idea of what is involved and whether the business is for you or not. Believe me, doing a disco is more than just music and flashing lights

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You don't mention how long you have had an interest in D.J'ing?. No offence, but I know that I had several hobbies and interest's when I was 16, but they only lasted for a few months, but the interest in D.J'ing was there for years before I actually took it up, so I knew that it was worth my while to persue it.
My advice, is to try and get at least one gig, if not a Saturday job with a D.J local to you, then you will at least have an idea of what is involved.If possible carry on using your friends Pc based system, although I still maintain that it is better to start with, and use real decks and Cd Players.
If the interest is still there after six months, and attending at least one gig, then it is worth investing in some gear.
When buying the equipment, it is a good idea to buy something that will serve you out on the road as well as at home, so avoid buying the cheapest items that you can find, because what is usable at home, may not work / sound okay when you take it to your first gig!.
Ideally CD is better for taking out on the road, but you may wish to use turntables at home. I'm not sure of what the prices are like for gear in Australia, so I am unsure of what to advise you to buy to suit a budget. Maybe you could visit a Sydney Dealer at some point?. If you do get the chance then make a note of what Mixers, CD / Turntables are available and within your budget, then I'll advise you on what would suit you best.
Maybe at this point you should decide, given your location, whether you want to become a Mobile D.J or just as a hobby at home. If you just use it from home use, then you are looking at a considerable saving on buying the euipment, in fact you could get away with using a p.c based system at home. However if you want to make a go of being a Mobile, then it is false economy to buy gear which will work okay plugged into your home hi-fi, but not work correctly when used in a hall!.
Even if you decide to use a PC based system for mobile use, then you'll still need to buy Amplifiers, Speakers and some sort of lightng. But I still think that you will be better off buying a conventional CD / Turntable based system first.