I will put in another vote for the Fuji line of rangefinders. They are a little bigger than a Mamiya 7 but built like tanks, half the price and still smaller and lighter than other medium format SLRs. They basically come in 4 flavors; 6x7 and 6x9, wide angle and normal. You'll need a light meter but a lot of times I just use a compact digital camera to judge the exposure. I'm actually getting pretty good about just guessing the light.

Traveling with just a single camera and a single lens is elegant and appealing. But, if you want to take a 4x5 then do it. If Werner Herzog can lug a boat thru the Amazon then what's backpacking with some extra gear? Personally I use a Busch Pressman. It's like a Crown Graphic but the back rotates to vertical, which is helpful because I do a lot of portraits.

The real issue, if you are traveling for such a long time, is what to do with all the film. I would be [am] paranoid about getting my stuff stolen. I live in Buenos Aires and there are two decent labs here that process large format [Buenos Aires Color and Megaphoto]. You can also buy 4x5 sheet film for about 50% more expensive than New York prices.

I suppose you can Fedex film back to someone in the US, altho I'd be nervous about this. I guess it's pick your poison.