Multiday outings, backup and photo preview.
This translates to high battery capacity, reliable hard drive and a good screen.
I would recommend you buy a 500Gb Lacie rugged external hard drive for backup:
About 150$ 250 Grams
For power I recommend a Brunton Solar charger. This will power your camera, laptop, phone, mp3, etc. out in the wild:
Brunton Solaris 26 is about 280$ 955 Grams
Finally, the laptop which has to be lightweight and have a good screen.
What makes a good screen?
-Large view angle.
-Large contrast ratio.
-Preferably LED backlight so you don't need to wait for screen warmup.
-More resolution is better.
I think the SAMSUNG N150 is pretty good. It claims that it's screen is non reflective even when light is directly infront of it, it's LED backlit, it's lightweight and claims a 8.5 hours battery life.
Cons: 1024x600 10.1inch screen isin't exactly big and it's keyboard is pretty cramped.
It's about 420$ 1.24 Kg
Total Cost: About 850$.
Total Weight: 2.485Kg/5.48lbs
In my opinion this setup will give you the best weight, battery life and storage you'll be able to find. The only thing lacking here is the screen resolution but for checking focus and displaying pics it should be fine.
This is a netbook so don't expect loading photoshop on it and doing photo editing.
They are other laptops i've found that have a much better screen for under 999$ and can run photoshop but their battery life is mediocre and they weigh a lot more. For example the vaio F series and the lenovo Thinkpad T series.
I recommend you have a look at them just in case you fall in love.