Hi,

I would recommend Osprey Packs too.

anyway, try some packs on your back WITH WEIGHT ON! in the store, let the sellsperson help you, and try about 3 different packs at about 3 different stores to decide.

Assuming that Emma is for a women, I'd reccomend a Women specific model since the fit is better.

about volume, for few months you would need about 55-70 liters (3300-4300 Qubic Inch) depends on the season you're heading and how light is your gear.

For further information I recommend www.backpacker.com with great tips and 'gearfinder'.

* You didn't mention where you're coming from, so I can't realy recommend stores or brands that are local and cheap for you.