I considered an IGH however went for a derailleur when I considered the following points:

- more gears - I change gear a lot to keep my cadence comfortable and I hate being in a gear too high or too low
- I rarely do maintenance on my derailleurs and they seem to work ok , any maintenance is pretty quick and easy (usually just a tension adjustment - 10 mins tops)
- derailleurs are lighter, cheaper, and provide a better and more customisable range of gears
- I like drop bars and STI style shifters (although I know that there are some out there now for the alfine IGHs)

I think for a long journey like yours, I'd be looking at a comfortable road bike - more of a sportive geometry. This will mean you are a little more upright so you can see better, but also will allow you a comfortable journey with drops for headwinds.

Daven