Engine coolant

[Orangebob]

Hi all, just a quick one, basically my coolant expansion tank emptied itself in the boot, looked through loads of forums and will be fitting new tank myself on Monday. My question is what do I fill the tank with? Can I just top it up with xxxx or does it need draining and starting fresh so as not to risk creating gel like substance others have reported by mixing 2 different antifreezes? Lastly how much antifreeze do I need for topping up or to fill full system? Sorry if this seems a bit of a noob question but the closest I've ever been to any mechanic work is changing a wheel, thanks in advance, Phil

P.s. car is a 2001 Boxster S and the coolant fluid was pink if that helps