How much do you want to spend?
Will this be used just this one time, or do you plan on using it for other things later?
When shopping for a pack, it is critical to try them on first to ensure a good fit. I did probably a hundred hours of research before buying my pack online, and still did not get a good fit. There are just too many variables that need to be considered and not enough dimensional information provided by the manufacturers online. Even if it was all there, it would be a real challenge to be able to visualize how it would sit on your back. If no one sells this stuff locally, you can do your research online, buy from someone like REI (which has a good return policy) and exchange as needed. B&M REI stores will fill each pack you’re interested in with weights and let you walk around the store so you can get a better idea of fit.
A pack should transfer most of its weight (~80%) to your hips. You don’t want the straps to rub your neck or shoulders. The pack should fit the curvature of your back so that there is little to no gaping. The hip belt should fit snugly across your torso, below your natural waist but above your hip bones. It should not feel top-heavy, but balanced.
Taking the time to get the right fit is totally worth it, especially when you’re dropping a couple hundred bucks and planning to wear it a couple hundred miles at a time.
I don’t think any packs are 100% waterproof. Packs are typically made of ripstop nylon that is not waterproof. You can buy a rain fly to go over your pack (also made of nylon, but coated with waterproofing chemical), and this works pretty well. You can also just cover your pack in something like a garbage bag. Or, because nylon is pretty quick-drying, you can simply pack the water-sensitive items you’re carrying in dry bags…which is what your chosen pack essentially is. That pack looks like it would keep water out for the most part, unless you went swimming with it. I’ve swam w/ dry bags, and they leak after a while, but do keep things surprisingly dry overall.
Another thing to consider is capacity. You don’t want to have things dangling from your pack exterior all over the place. You also don’t want to load it up with more weight than you can safely carry.
And finally, weight. The weight of the pack will always be with you so the lighter the better, but probably fit and comfort is more important (i.e. better to carry a little more and be comfortable than carry less and carry the whole load on your shoulders instead of hips).
I was also under the impression mountaineering means climbing and that walking on or amongst mountains is really still hiking…Searching, I see it defined to be both. Can anyone clarify?