I am also looking at rucksacks. I have a Vango Sherpa, 75 litres, which weights almost 2.5kg. I used that in the Beacons for a week's walking - although food and drink was so hard to come by that when we found a campsite where the owner gave us the run of his veg patch and also sold us meat (home produced) from his freezer, we travelled light thereafter.
I really don't know how big a rucksack I am going to need. Tent, downmat, quilt, clothes, cooking stuff...