Like most things in life... it depends on what you want to achieve and how much money you have to spend.
You can install only the PSS9 kit and leave it at that. But if you lower the car using the ride height adjustment on the PSS9's, then you'll also need adj rear toe links because the stock ones do not have enough adjustability to account for the lower ride height.
Of course, you will also have to address old or worn out components that may need replacement due to mileage or age. Top shock mounts (front and rear) are common to replace when the PSS9's are installed. As your shop suggested, there could many several other components that may need replacement. It depends on the condition of the components.
Next is sway bars. If you upgrade to the 996 GT-3 bar in front and the Tarrett bar in the rear, then you'll need front and rear adjustable sway bar end links because the stock ones do not have any adjustment range. I wouldn't install rear adj LCA's unless you find that you need them.
Next step is lower adjustable control arms in front. These will allow you to dial in enough neg camber to keep the tire flat with the suspension compressed on the track. Without these, the car may not have enough neg camber and you'll tear up the outside 1/3 of the tire on a consistent basis.
At this point, you basically have the Boxster Spec suspension and it is a very good and well tested system with the PSS9's. This will give you an excellent street/track suspension with reasonable adjustability. Also, there is a lot of experience with this system so you can get good apples-to-apples input on alignment and setup recommendations.
Here is what I installed:
http://986forum.com/forums/general-discussions/35845-suspension-upgrade-week.html (Adj front LCA's were already on the car).
Beyond this, you can add more components but I'd suggest that the performance returns are going to start to decline as the cost continues to increase. Hence, this might be a good point to stop installing new parts and do some driving and see where you think that the suspension isn't performing and then address that later.
6-8 hours for ride height, corner balance, and alignment seems high. A good shop should be able to do it in 1/2 to 2/3 of that time.
Also, shop around for components. RSS makes awesome parts but they are expensive. Take a look at Tarrett and other brands. The Tarrett front adj LCA's are $715.
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