IMO the PSE is the 'must have' accessory on the 996. It is a very clever piece of kit and really enhances the character of the 996 without being particularly noisy.
If your car is fairly new (ie still under original/extended warranty) then you might be wise to have it supplied and fitted by your OPC - c.£1350 without switch, c.£1,750 with switch (after discounting). IMO the switch isn't necessary as the default setting is 'on' and, ultimately, the sound level is controlled by your right foot.
If your car is older, then you could try some of the independents, but you would be lucky to find a secondhand PSE. Alternatively you could consider other makes, but these may not be as refined as the PSE system.