I have a couple of large Lowe Alpines (80 and 90L I think) which Mrs S and I used for a month in Thailand.
Good value for money as a brand and nothing really sub-standard about the quality.
Consider security - does the pack make it easy for pickpockets? Can a lock/mesh be attached easily and securely?
You will want a day pack as well for when you are out and about. Some rucksacks have zip off day packs. You can also get say an osprey OMM pack that will fold up nice and small.
Ones with seperate access to a bottom compartment are good for storing bedding in so you can get it out without emptying the bag.