Good news, it doesn't sound to me like you need a coolant tank. Never go to the folks that quoted $1200 plus the parts again. They don't know what they're talking about and probably are incompetent.
In big metro areas, you have many choices of shops. I look for the worst organized ones with loads of cars in the storage yard. Busy and popular.
Welcome to the part of the Boxster world where you have to worry about the puzzle of the cooling system. The coolant dump is part of the overflow system--poorly designed as it is. My ancient Dodge Dakota never dumps coolant out the coolant reservoir. Neither does my ancient Prius.
Nice thing about topping off with distilled water is that it's cheap and compatible. Bad things are that it doesn't show up in the reservoir very well and it dilutes your coolant. Great in an emergency, but only as a get it home solution. Next time it's low on coolant, replace the coolant level sender. It's quick, cheap, and easy. I'd get some sort of OAT coolant--Porsche, Audi, or VW brands are all compatible. Even Toyota coolant is compatible. (And red!) And keep a jug of mixed coolant in the trunk for topping off. Don't overfill!
There is an updated coolant cap that's supposed to help. I had the green updated cap, my local Indie shop GAVE me the black one! (I wrote a glowing Yelp review for him!)