Peugeot 308 Diesel Problems
Peugeot 308 2013: Is my Allure a lemon?
We don't have a definition of what constitutes a lemon in this country, but I wouldn't call your car a lemon. Most of the problems you've had could be called maintenance issues, like the battery, brake rotors, shock absorbers, tailgate struts. They're not major issues, more frustrating ones. I wouldn't be trading it in because of your experience. I would recommend that you find a specialist Peugeot mechanic to service your car; you'll save heaps by doing that.
Peugeot 308 2012: Failing turbocharger
I would think the problem is not an oil pressure problem, but more likely an oil supply problem to the turbocharger. It would be a severe problem given that the turbocharger is lasting a mere 30 km. I would expect that he has changed the oil when replacing the turbocharger, but that would be a must if he hasn't done it. There are no 'lemon' laws in Australia, so you have to battle that one alone.
Water in Fuel
HOPEFULLY you have a receipt showing the date and time you bought the fuel and, armed with the report from the dealership, go to the servo and show them they have a problem that may affect other motorists, and go to the fuel company and seek compensation for the cost you have incurred.
Hell and high water
A HOPEFULLY you have a receipt from the servo showing the date and time you bought the fuel, and armed with the report from the dealership showing the water content, you need to go to the servo to show them they have a problem that might affect other motorists. Also, go to the fuel company involved and seek compensation for the costs you've incurred.