As mentioned above...
1. Aftermarket Depo Headlights work fine - beam pattern is fine. Sure eBay have plenty for around £200.
2. Adding a HID kit will not fail a MOT unless they are levelled wrong (same as all lights), the kit is faulty and not putting out light correctly or your tester is a tool
3. Best light output IMO is 4300k...unless u want a bluer light then go for 6000k
4. Can bus error free means nothing - similar price anyway if it makes u feel better.
5. Only thing u need to do is have xenon without shutter programmed in VCDS..
6. Slim ballast is easier to attach to the light..I have a bigger ballast but it is screwed in and also stuck with no more nails pads - no problem
I hope this reassures you. It's a top upgrade..!