Thanks guys for the feedback. I'm aware of the level of preparation required for this, and that is one reason why I have given myself almost a year before I will attempt it. The following 10 months will be used to prepare the route in detail, I will have a daily itinerary with mileage etc., providing me with an accurate understanding of what to expect during each stage of the route. I'm also focusing my efforts on raising as much publicity as possible, particularly here in South America. I agree that 26" wheels are the way to go, purely because if something was to happen while on the trip I won't be completely screwed, although having spare parts ready in Rio for my girlfriend to FedEx out if need-be is also sensible.
Since my original post I have had a change of plan, and decided to fly back to UK for xmas and to buy a bike. Although it is on the heavier side, I believe the Surly Long Haul Trucker (very similar to the Thorn Club Tour used by Scott Napier), while carrying as light a load as possible, is suitable. As was mentioned, reliability is of most importance, and I think the very strong LHT, if fitted with the correct gearing and 26" wheels, is one of my best options. I just need to try to cut weight where possible. I have discovered, from reading endless forums, that there will never be a unanimous agreement on the 'best' bike - "a camel is a horse designed by committee" springs to mind! Also, although I don't underestimate for a second the importance of the bike, the majority of this challenge will be physical and mental, the bike is only so important.
I would still value your views on the LHT, check out the Thorn Club Tour for comparison! Another option, although probably not the best idea, would be to upgrade my Dawes Horizon 2010 to make it suitable.