First lesson I got from my friends who used to travel around the world a lot, don't bring anything you don't mind losing.
Simple lesson, hard for american style consumers to grasp, I used that method and it was great.
You can buy a used lowe alpine outback 70, which is a great pack, about 4 pounds, cordura, old style, no fancy things, it's a bag with side pockets and a top. Has thick solid dual aluminum stays. Large lumbar pad, well padded straps.
Real internal frame, rock solid, I paid $15 US at a fleamarket for mine, easy to find online too. Because it's so minimalist, even with the heavier materials, it's reasonably light.
Will easily carry your weight you want to have, and will not stress you out with losing expensive gear in the hills of peru, which are not the same as the hills of the Sierra Nevada's, where mchale and other expensive packs are reasonably fine to use, if you can handle the cost.
The other practical suggestions here are also good, old externals, though I think I prefer, having used both old style external and internals, internals.
Cordura is indestructable, and the weight difference between a modern and expensive pack and the outback 4 just is not very much in any real sense, about 1 pound maybe max, but the outback is made with heavy grade cordura. The outback stands out to me because it's so minimalist, really all it is the materials required to constract the bag, straps, pad, and frame, and nothing else.
Besides, you are going to be a missionary, and what did Jesus really feel about excessive consumption and all that in the end, right? Doesn't hurt to practice what one preaches, tends to impress those you are preaching to. The least you need to do the job is the right choice, and getting rid of the consumerist mindset is the best place to start on finding that least. Wasn't he the guy who wandered around deserts with nothing, and has basically no possessions? And who knows, maybe if you're really lucky, you'll be the one who gets converted...