As of January build F20s will come with DAB, auto lights & wipers and xenons will now be a £500 option rather than only available through the £1140 visibility pack. I'd imagine your car will be built January or after as mine was booked over a week ago and just got in the last December build slot. The base price of the car will however be increasing to reflect the new standard spec.
Sensors are indeed only an option. Might as well add the driver comfort pack if you think you need sensors. I need them due to the parking I use and this is what I did. It includes cruise control as well.
It seems that both nav and xenons will add to the GFV (since I figure you are going the PCP route). Not sure how much exactly. When I added the visibility pack the GFV rise meant the option was only costing another £7 per month. The best thing to do is add everything you think you want. Get a price and if it's not where you want get your chosen dealer to run some other quotes chipping options off.
I never went for nav as I have absolutely no need for it. Nice to have, yes but would only ever be used a handful of times. Xenons will get used every day and from what current owners are saying, the halogen units are like driving by candle light. The fact the top half of the dipped beam light is used for an indicator seems to be the issue that makes them bad.
For good info on the F20 get yourself over to babybmw. Lots of info and experience to help you make decisions. Few familiar names from here over there as well.