For starters, I'm no mechanic. That said, I read a lot and have done a number of repairs on the Box I have owned for ~14 years now, much of the info derived from this forum.
When it comes to refilling coolant, seems to me that using a vacuum filling system is far away the best approach. That said, if you want to go the other route, read
insite's recommendation in this thread. (He's a member who, unfortunately, doesn't frequent this site much at all anymore. I say "unfortunately" because it seemed to me he did a lot of his own work and had a pretty good idea what he was talking about in his comments/recommendations.)
http://986forum.com/forums/boxster-general-discussions/28548-cooling-system-woes.html