I've been in a similar position as you, somethings that you seem to have gotten confused about...
The house will only become your PPR if
a) Your parents gift you the property now, and you pay the tax due, so approx. (500-335)*0.33 = 55k
b) If your parents do this, and go into a nursing home within 5 years, the value of the house will be taking into consideration in calculating how much they owe - which could cause cashflow issues for you.
c) if your parents to gift and go into a nursing home after 5 years, then the house is exempted etc.
The only way you can get the property without a tax liability, is to be living in the house another year, and then inherit the house when your parents pass, and only at that point, live in the house for another 6 years. So if your parents live for another 13 years for instance, you would then have to live an additional 6 years (so 19 more) in the house to receive it with no tax liability, and that is based on current laws.
If the parents go into a nursing home and a charge is put against the house, then you will need to pay this off at time of inheritance, which could cause cashflow issues.
All in all, given your circumstances of two parents still alive, there are a number of factors that you need to take into consideration, some that you might not even think about now - lets say in 2 years time you meet someone and they don't want to live with your folks, what then?, or let's say the parents gift you the house and you meet someone get married and it doesn't work out and take half the house - your parents could potentially end up out of their home.