1. At the beginning i will do 24h trips, but the backpack needs to be fit for 72 hours in terrain. I will use minimal gear, very few comfy stuff so I guess 35-40 liters will be enough, yes or no?
2. The pack should be really durable, even if its going to mean it will be heavier and less comfy.
3. Im not a fan of military design. Molle, camelbacks - its all useless to me. The simpler the better.
4. Im don't want to spend much. This first pack is supposed to show me my preferences and allow me to start out, not last for lifetime. I will change it anyway when i get some experience.
1 is what you expect, but buy your gear first, and use it, then see where you stand sizewise

I used to think the same way as you about 2. After actually going out, i've come to the conclusion that 2 is far from true anymore

3: a camelbak can be very handy, really helps you keep hydrated when you're hiking. You'll more quickly reach for it, than getting your drinking bottle out of your pack (or even out of the side pockets of your pack, it's just a pain in the bottom sometimes

Just get your gear first, and then get a pack which fits and carries it well. Get a good pack, which transfers the weight well to the hipbelt, the shoulder straps should only stabilize the pack, not carry any weight and be sure you can try the pack on before buying, with a load in it, for a decent time, so you can feel how it actually carries. Your back will thank you later
