Tynesider, if you like Osprey packs then the Atmos 50 would be worth a look. Other packs in the 50 litre range I've found good are the GoLite Jam and the Lightwave Fastpack 50.
Any stove should work fine above 3,000 metres. It's temperature rather than altitude that can make a difference. And because of the reduced air pressure gas canister stoves work better at low temperatures at high altitude than low altitude! There are plenty of ultralight little canister stoves like the Primus Micron, Coleman F1 Lite, MSR Pocket Rocket and Optimus Crux Lite that are excellent. If you'll be using the stove at low altitude in sub zero temperatures then a heavier remote canister stove like the Primus Express Spider would be better.