I'd look at super light weight tents before bivis.
There's the Terra Nova Laser super duper lightweight one at 720g for a complete proper double skin tent.. it is pricey but there are cheaper versions of it. Much better than a bivi imo.
The advantage ofa bivi though is that you don't need flat ground – you can just curl up behind a rock/under a tree wherever you want.
My advice re food is take dried food – no point in carrying out extra heavy water in your food when you don't need to. There are some good ranges of complete dehydrated meals, or there's noodles, pasta n sauce etc. It's only one night after all 🙂
You can find water in the mountains easy enough, and it's clean enough to sterilise with tablets. For tabs get Puritabs – silver based so completely tasteless, but they are more expensive than chlorine ones.
It is definitely worth paying a lot of attention to weight when you plan a trip – struggling around with 12kg on your back will most likely impact your enjoyment a lot more than say, 5kg.
Edit: Oops, got carried away with the advice there.. sorry.