I don't think there currently is an all-in-one lens for FX format sensors at the moment. Sigma and Tamron alternatives to the Nikon are, I'm nearly certain, all for crop bodies. I'd suggest getting 2-3 lenses for photographing everything rather than one superzoom. An 18-55 or the equivalent FX format (28-70, I think) lens is good for landscapes, cars and portraits where there is little room, a longer telephoto (think 55-200 for DX or 70-300 for FX) for wildlife and further distance portraits, and possibly a 12-24 (I know Sigma does an FX version of this, and it's been getting fantastic reviews) for extreme landscapes. However, if you to get the D700, make abolutely sure the lenses you're getting are FX format; try them at a camera store if you're not sure.
To answer your final question, a higher ISO at night is usually not that useful, so long as you have a tripod. Simply set the camera on the 'pod, choose a 2-second self timer, and shoot. Higher ISOs are only helpful for indoor shots where you need a faster shutter speed, like 6 year old kids running around like maniacs, but where you don't want to use the flash.
Basically, I'd recommend the D300 for the kind of photography you're talking about. It's a perfectly capable camera, and the high-ISO performance is already stunning. Or you could get a D80 and spend your cash on some REALLY nice glass.
Whatever choice you make, I hope it's the right one for you.