I had to have all the components, except the pump, replaced last year. I watched my mechanic replace them all, including hoses, from the top without any issues. The intake manifold comes off easily and quickly without having to take the extra step of opening up the front which will extend the time, and effort, required to do the job.
The problem with the SAS is that when the main on/off valve fails it fails in the open position so the system is still doing its job but full time instead of a couple of minutes when first started. The open position also allows hot air to flow backwards through the system and eventually fries all the other components.
You might start by checking the main on off switch which sits right on top to see if it has failed. If it has plan on replacing all the components, if not it may be one of the other switches/diaphragms or a hose. Since the SAS is a common failure issue with Boxsters I would go ahead and do it all.
