It's a 7 year old vehicle. It's now an old car as such.
Things wear out and break. Often.
You say you know nothing about cars. Bad sign.
You need to learn to read and listen for signs of a future problem. How often did you lift the bonnet and check check check the fluids etc. etc. etc?
Choice is fix it or sell it as it is, and purchase a vehicle that a proper mechanical inspection has been done.
As unfortunate as it is... and a blunt as the responses are.... its the truth of the matter.
A car is a complex sum of mechanical and electrical parts, that requires maintenance and sometimes repair. Everyone has ended up with a lemon at some stage in life, which sucks balls.
There's a reason I buy new cars now, A) because I can now afford them and B) I couldnt be arsed dealing with the **** that sometimes comes with pre-owned (and sometimes abused) cars.
Chances of getting any recourse is practically non-existent... and buying a used $10k - 7 year old car is always going to be a gamble... you also have no mention of how many kilometers were on it when you purchased it or how many km's you've clocked up.
As for your braking issues after new pad and rotors being installed, I would be looking firmly at the mechanic that carried out the work. This is nothing to do with Holden manufacturing or the second hand dealership you bought it from.