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You're not going to be running at full settings on a $600-700 machine unless you're playing at 1280X720 unfortunately. If the $700 needs to include windows, or worse, a monitor, speakers, keybo, and mouse, the prospects get worse.
Lowest spec I'd go with for a gaming machine to be able to play at high/ultra settings at 1080P (and sometimes 1440P) would be something like an entry level i5 maybe an i5 6500, a cheap H110 motherboard, 16GB of RAM, maybe start off with a 250GB SSD and add a 1TB HDD later when funds allow. Round things out with an RX480 or GTX1060, a decent 500W power supply, and an affordable case.
And we're talking $700 AUS. While the SSD is nice, it isn't really needed for games. Otherwise, I think Theocnoob hit the minimum for the OP's stated goals. Depending on the games, 8 GB of RAM wouldn't be a hardship either.
This is about the best I could get in 750 with windows dude. I live in Aus too, you'll need to cough up a bit more cash if you want something to run things at 'full settings' (Resolution pending) Overclockable and upgradeable also don't really come into play here either.
SSD for speed, add a 1tb drive later for 70 bucks.
That's a decent build (The PSU frightens me. Not your fault, Mjolnir. Budget is what it is). However I would opt for a 1TB HDD for the same price as that SSD you have included. 120GB is not enough space.
That setup will definitely not be playing games on ultra settings (except maybe DOTA2 and LOL) though. I think the OP needs to temper expectations or double the budget.