Breeze, what do you intend to do with your Boxster? If your only looking for "reliability" why do you need +300HP and an engine swap? If you intend to do stuff like autocross and DE, you also need to invest and budget (and keep the Boxster's balance) in for safety items like better braking and suspension. Do you intend to just do fun & spirited country road cruises on the weekends? You'd still need more than just extra HP's, IMO.
Is your clutch up for the task? Cooling system? What about that exhaust note?
Lastly, how much have you, or do you plan to, invest in yourself, with regards to 'driver skill development'? Are you part of the PCA? Do you work with any of the Club's driving instructors? Keep this in mind before you start to plung $$$$ into the Boxster. Invest in yourself too.....skill development......and safety.
I am just trying to offer a different presepctive, and challenge your thought process.