As for panniers, they should be made of tear-resistant fabrics, because they can always get caught on some protruding rock. The best tear-resistant fabric is Cordura, but there are other quality fabrics as well, especially rubberized ones.
Basically, when preparing for any bike expedition, there only two pannier producers worth considering: Crosso and Ortlieb. Crosso panniers are in many ways based on Ortlieb products, except for the carrying system, which is very simple, but effective.
In my opinion, however, Ortlieb panniers are still unrivalled. I suggest checking the website of Cyklotur or to find out why I think so. Roller panniers from Ortlieb are 100% water resistant – you can throw them into water and they will not soak through. All Ortlieb panniers sport a great carrying system. You can adjust it to almost every rack in few seconds. For this system there are available all spare parts. Moreover, they have welded seams which make the panniers completely waterproof. The fabric is extremely durable and when it gets worn, you can patch it from the inside, even using tyre patches. Additionally, Ortlieb gives a 5 year guarantee on all its products.

A few more tips

-Do not purchase black panniers – after a few hours of cycling under a scorching sun, the contents will be roasted.

-The fewer zippers, the better. The thing about them is that they sometimes break. The best solution is a roll top closure (like in Ortlieb Roller) or a lid closure – like in a rucksack.

-I recommend side panniers because they are easy to mount, remove and carry around. They also offer better access to the luggage. And at the top of the rack, there is enough space for a tent.

-Last but not least, make sure that panniers have stiffeners. Without them, the corners will rub against the spokes. The stiffeners should be made of plastic rather than cardboard, because in case of rain, the latter will soak, and consequently the pannier will get deformed or even completely destroyed.

-Do not delude yourself that panniers are really waterproof (apart from Ortliebs, which have welded seams). Water will always find a way to get through. Thus, all items should be packed in plastic bags.

-When everything is stored in separate plastic bags, it is also easier for you to find what you are looking for.

-The pannier mounting system is very important. Expensive panniers usually have solid buckles which prevent them from falling off, so it can only happen to side panniers and not to panniers shaped like a reversed letter U. As for side panniers, they should have attachment hooks or a tightened bungee strap with a hook, which can be attached to a mounting hole in the rack or any other part of it.

-When packing up, put the heaviest items at the bottom. It will lower the centre of gravity, making the ride more stable. The luggage should be placed symmetrically, one-third of it being stored in front panniers.

-Your shoes must not rub against rear panniers during the ride.

-You should put some reflective elements on the panniers, preferably 3M reflective tape.

-It is good when panniers have covers which protect them from dirt, and to some extent, water. Ortlieb panniers do not require those, however.

-Using front panniers makes the ride more comfortable. When climbing uphill, the front wheel does not go up all the time. Weight is taken off the rear wheel, decreasing the risk of distorting the wheel and damaging or breaking the spokes.