So, I should get an APU, zip ties, razor, Phillip's screwdriver, ODD and SATA cables? I already have an OS, monitor, the PSU comes with the case and I have two 1 TB internal hard drives lying around. So all of the things that you have listed is good for my PC? Also a graphics/video card?
I have a hard time telling if people are screwing with me, or genuinely asking questions. If you take offense to what I'm about to say, I apologize in advance.
What is powering your logic? I cannot understand exactly what your end goal is, how you intend to get there, or whether or not you even understand what it is you are asking.
1) The PSU that comes with a case is universally scrap. You should never trust all your electronic equipment to whatever power supply could be bought in bulk as cheaply as possible. Barring taking that advice, make sure the PSU has all the necessary connections. Sometimes cheap PSUs don't come with 6/8 pin connectors, making future upgrades impossible.
2) I cannot make this clear enough. Licensing an OS with MS is more complicated than people think. You likely own a computer now with an oem copy of the license, and that cannot be transferred between computers. The only way you have a copy of the OS right now is if you've got a retail copy of the OS, bought a new oem license, or ar doing something not condoned on this forum. Please, consider this before leaping to conclusions.
3) The APU isn't a standard CPU. First, you're going to have to get a motherboard that supports socket FM2+. Next, you're going to need to decide on the level of APU you want. Generally, I build everything else on the low end of possibility and then plow all remaining funds into my APU choice when I'm building one of those systems.
4) If you want to use discs you'll need an ODD. That includes installation of an OS, older games, and any other media (movies, music, etc..). It is a 15 USD expense locally, so I include it when I build. Even if you only use the drive during installation, you can make backups of the system and burn them to disc so that any failures won't require starting over from scratch.
5) SATA cables are pretty much a necessity. Each drive (again, HDD, ODD, and SSD) needs a SATA cable (aim for III for compatibility, but II will be fine for all mechanical drives) to transfer data and a power cable from the power supply. Your motherboard is likely to have 1 or 2 included, unless it's absolutely lowest of the low end.
6) The screw drivers, razor, and zip ties should be part of everyone's basic tool kit (along with WD-40 and tape). If you really don't have these things you should ask if you're ready to take a risk with several hundred dollars worth of computer hardware. Again, not a judgement but you should really ask yourself this question before you get in deep.
7) I'm going into caps lock here, so it is apparent.
DO YOU HAVE THE SKILLS TO BE DOING THIS, AND IS YOUR BUDGET REALISTIC FOR YOUR GOALS? You've got a very low budget, at which most people can only get econo-boxes. Not gaming rigs, not media editing PCs, just econo-boxes that may play most games without many visual features enabled. What you've budgeted for is generally more cost effective to buy from a maker, and you'll be getting some sort of warranty with it. Assuming it is just for gaming, why not just get a console? While the PC master race pricks will be taunting you, gaming is about fun. If a console is the most fun per invested dollar then there's no shame in getting one. Assuming you want a PC that will play big new games, you've priced yourself out of that market. Seriously, this is where my brain starts to hurt. Between not understanding our comments, seemingly ignoring the body of the responses for specific key words, and generally demonstrating a lack of background knowledge I'd conjecture you don't have any experience in building or English is not your first language. In the case of the former, why not dip your toes into the market easier by buying a used computer that you can upgrade? Assuming the later, it's hard to communicate to you in a manner that won't either be patronizing or be misconstrued and potentially cause problems. Nobody here wants to give you bad advice, but it's difficult to get correct information to you if you interpret things without understanding them.
Edit:
So my US brethren understand. 1 USD is currently trading for 1.24 AUD. This means that a 400 AUD budget is equal to a 321.50 USD budget. Once you add on an increase in good pricing, and the local taxation, this is a very difficult build. If that wasn't enough, you have someone on the other end of the conversation that is just learning about building. Honestly, this seems like a bad idea for a first time build. Too difficult, without enough experience, and potentially poisoning the well against future builds.
If nothing else, losing a potential builder to a build that is too ambitious as a first project, is a travesty I don't want to be responsible for.