Sounds like you're on the right track. I went to Peru a couple years ago and brought the 450d (aka XSi) with the 15-85, 70-200 f4 IS and 1.4x teleconverter. I also had a circular polarizer for the 15-85. I found this kit to be quite versatile. Depending on what you primarily intend to shoot there could be better setups, but this one gives a good focal length range for a very diverse country! It is also not overly heavy or bulky (which is definately a concern when hiking to Machu Picchu). In the amazon, longer lenses are very nice, but keep in mind that it is often very wet (it is a rainforest) and can be quite dark under the tree canopy. So unless you can move up to the (expensive) tele's (ie the 300 2.8 I suspect would be great there) then this kit will still give great shots.
Another consideration might be to take a small travel tripod. I used a gorillapod with a manfrotto ballhead. I find these great for travelling as they don't take up much space or weight but still allow tripod shots (with a little creativity in finding something to hang the gorillapod from).
For a bag, I took a lowepro toploader which fit the camera and one lens mounted and the other lens I had in a separate lowepro lens case (which could attach to the toploader). I also took a regular daypack which would fit both the toploader and lenscase (along with water, food, jacket, etc). This had the advantage of being able to carry just the toploader (with the camera and one lens) or attaching the lenscase to bring the other lens or carrying everything in the daypack (which also conceals the fact that you have an expensive camera with you).
Hope you have a good trip.
Stephen





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