As neuro said, the wider the better.
What's the budget?
5D2 will get you 24mm wide, and maybe cost you $2k or so.
16-35L will cost you $1600, if you take care of it you can get at least $1400 back if you sell it 2nd hand when you get back. On the 1D2 you'll get 21mm wide, or get a 5D2 as well, and sell either/both on your return.
Zeiss 21mm Distagon is $1800, not sure about the resale value but shouldn't be much worse than an L lens (although, parting with it will be harder). 21mm on 1D2 is 28mm, that's not very wide these days.
EF 14 f/2.8 L is $2300 (18mm on 1D2). You can't zoom but you can crop, if you have to.
Those 3 are probably top on most people's (FF) list of landscape lenses. Not cheap, but worth it from everything i've read.
1D2 is only 8MP though. Is that enough? You could buy a lens and a 5D2 if you've got a lazy $3-4k lying around. Selling an L lens after a few months of use will cost you maybe $1-400, depending what you buy and sell it for. If you buy 2nd hand you may even make a profit. The 5D2 will do you very well, probably better for landscapes than the 1D2. Get a 2nd-hand one now for $14-1700, you could probably sell it for the same loss as an L lens after a few months (ie, not much).
£1695 seems a bit steep though. That's $2700. Don't shop at Jessops. I can't check the prices in squid directly, but try digitalrev.co.uk, they've got the 5D2 for AU$2070, which is £1400. Or shop around more, even Tottenham Court Rd should beat that, a new 5D2 should't cost more than that online, used maybe down to £1k.
As for the sealing, probably up to someone else to comment on the 5D2 specifically. But any camera can survive anywhere if you treat it right. In the hotel (with Airconditioning) put it in a sealed plastic bag, go outside to the humidity, and leave it in the bag an hour or so, then nothing will fog up inside. I've used my 7D and 70-300L (both very sealed) in the butterfly garden at Singapore Airport, went straight from the inside aircon to outside with no plastic bag, fogged up straight away. No amount of (reasonable) weather sealing can stop that...