You need to consider your budget. Me, I'm a total amateur so anything more than a 400D is a waste of money.
When you say scenic photos do you mean landscapes? If so, you may want to go with an ultra-wide if you are shooting with a cropped-sensor camera such as the 400D.
With the 400D, an ultra-wide zoom like the Canon 10-22 if you can afford it. Alternatively, the Sigma 10-20 is much cheaper and produces results no worse to a less discerning eye like mine.
Bear in mind that both of these lenses are specifically for cropped sensor cameras and won't work on full-frame cameras...so this is something to think about when it comes time for possible re-sale or the next generation of prosumer digital cameras which may abandon cropped-sensors.
The 400D has 9 auto-focus points. The D80 has 11. The 1Ds has 45.
"Fast low light auto focus". If you're doing landscapes, you'll be using a tripod and cable release with more often than not hyperfocal distance focusing. Fast auto focusing isn't really a concern. So perhaps, a more detailed explanation of what you actually having in mind might enable better recommendations.
Also bear in mind, the lens is obviously crucial in determining auto-focusing speed and dealing with low-light...not just the camera body.
1000s of informed end-user "subjective" reviews and ratings of just about every lens you might be considering:
http://www.fredmiranda.com/reviews/
A decent dealer should let you test out a lens to varying degress before purchasing.
If you're just starting out I personally wouldn't bother with a body any more expensive than the Canon 400D. Even then, it probably has too many features and automation to prevent a total newbie from focusing on properly learning the basics without being distracted.