My input here may be totally useless as our product availability is different to yours. However here are my thoughts.
First determine the capacity range you need. If you are going to be in tight scrubby country you don't want anything hanging off your pack. It should all be inside so as not to get caught up. IMHO, anything above "daypack" size MUST have a good waist harness. A few hours of load on your shoulders will confirm this. Sounds to me like around 50 litres is what you want. (I think it's around 64ci to a litre) So the next and most important thing is the harness. If the store won't load the pack up for you and help you fit it correctly, then they aren't serious about selling packs. It's always impressive what you can learn from someone who knows their stuff in a good store.
One Planet are an aussie maker and the harness is the absolute best I have ever used. Lots of adjustment and shaped to fit. I've had the Strezlecki for several years and taken it from local hikes to Nepal and back and it's still like new. If they don't have them over there, just fly over here and pick one up!