Aether 65L

Start cheap, see if you even enjoy it. If you do, shop around for what's comfortable for you. That being said, I have been very happy with Osprey Aether 65L. I've used it on several 12 hour events that often involve rucking with 100# or more.

I use an Aether 65L for everything. Big enough for trips to Europe etc, but small enough I can use it for a weekend bag. If anything, I would go down to a 55L which would still be enough bag for most occasions. Or you get a 65-70L hyperlite that rolls down into a 40L basically when not full. Go get

I think that what is worth investing is a compact (1-2L) 0°C sleeping bag, a good backpack, a light and compact tent and clothing. Other stuff like a propane burner, cooking stuff, trekking poles, hygiene stuff (look for "travel size" stuff) and sleeping mattress don't need to be super expensive to

I'm more of a traveller than a hiker, but I did work in outdoor gear for quite a few years. If you are set on an Osprey, the Aether 65L (a little larger than you wanted) might work for you. I don't know how well it handles mountaineering but this thread has a bit of info on it.

I'm not sure what the military uses, but I've been rucking for over a decade with an Osprey Aether 65l. Heaviest I've loaded it was a pinch over 100lbs, mostly with sandbags. I go a bit old school and rely a lot on the the hip belt, so the weight is better distributed, and I can go farther, faster,