Not sure if you read this yet
Thinking of buying a Boxster ? - MUST READ !
but it is a good start.
Also, 150K is that miles or kilometres? Are there any service records? the '99 has a dual row IMS bearing so that is good but regular oil changes are still a must so some records would be nice.
PS the Upper Canada Region of the PCA is the largest and very active chapter of the Porsche Club of America so join the PCA and connect with some members there. You will get a wealth of knowledge.