Hi Ken, when I was in the market for the clutch bits I bought them from Euro Car Spares as they had (at the time) the best price on the original spec parts - they were a lot cheaper than Porscheshop and Design 911. However, things may have changed. You could always try Bert Gear too, I have done a few deals with him in the past. Best policy these days is to phone around and 'haggle!'
As far as the brake booster is concerned I would think the LHD and RHD cars had exactly the same units installed as all we have over here is an extension bar assembly across the car to the pedal box. Of course there are differences between years and models (according to the PET), but I would think you are pretty safe ordering a US spec part (if it works out cheaper with shipping and import taxes, VAT, etc. too?). Difficult to return if you have a problem under warranty though

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Hope that helps.