Trying to answer the questions you asked:
- Brightness is certainly an issue - but I find the stock lights to be quite good, so if at least that much illumination remained after the conversion, I'd be satisfied.
- Staying in the Porsche family for the light is irrelevant.
- Halogen per se is not critical, as long as the first answer is accomplished.
- Flat, or near flat, would seem best, but some (modest) curvature would probably be OK as long as it didn't protrude or appear bulbous.
- Something under $1k would likely have me do the conversion.
- Yes, I'd remove all existing components and mechanics.
- I agree with your objective of avoiding a light behind a dummy lens, but I can imagine an acceptable result that would use this approach.