1. Definitely but doesn't have to be dealer history as long as it is a reputable garage, preferably a VW Audi specialist repairer
2. Nothing wrong with high milers as long as it is 'proper' miles (e.g. used correctly, not smoked around town on short journeys, etc)
3. Aftermarket stereos are far better for functionality that standard fit
4. remaps are okay if the servicing has been carried out and it wasn't owned by a hooligan
5. Not sure, sorry
6. Worth checking the forum posts for further info