1) Am I right in thinking that the PCD of the wheels on my 06 JTDm is 98? If so does that mean any GT, 147 or 156 alloys should fit? I currently have 16"s fitted and would much prefer some OE 17"s or 18"s, but I need to know what I'm limited to.
double-check the offset of any wheel you potentially buy. for a 7~7.5" wide wheel, the offset needs to be around +38mm (ie: ET38). can usually be within a range like +35mm to +42mm, depending on tyre width; this avoids unwanted 'rubbing' of the tyre-edge on the splash guards, especially during full-lock of steering.
from memory, the standard 17x7.5" wheels - with 225/45 tyres - on my GT were +38mm.
916 GTV/Spider offset might be significantly different. can't remember.
2) Is it possible to retrofit HIDs, either from the dealer/scrapper or an aftermarket kit?
generally considered 'too hard'. for full compliance, you need:
1. projector lenses (not reflector lenses): the ones that look like a fisheye
2. headlight washers
3. self-levelling system for headlights
simply fitting genuine HID xenons to the standard reflector lenses are likely to cause excessive glare/flare to oncoming traffic, and can have a poor beam pattern on the ground.
fitment of the full lot would be extremely expensive. self-levelling is via sensors connected to the front/rear suspension arms. different wiring loom. bumper and washer mods.
best compromise is quality halogen globes, like Philips Power2Night (formerly called VisionPlus).
3) Is it possible to retrofit parking sensors.
as above, quite a few different aftermarket kits available. rather than drill the bumper to fit 4 separate round sensors, you can also buy ones designed around a long strip that is secured around the back of the bumper ('inside'), out of site.