I know this has been done to death on here but everyone seem to have there own opinions. I have a new braking system on my car with remanufactured calipers on the front and brand new ones on the rear. Brake fluid has never been in the system. I intend for fast road use and maybe the odd trackday.

Which fluid should I buy and how much??

5.1 is best IMO
How much depends on how much gets wasted while bleeding