When buying a pack, size is the main consideration - 75 litres is big enough for most purposes. There is not much to choose in this market - the current generation of internal frame packs all seem well designed and durable. Locally, Karrimor and is the best known brand with a well-proven record, but other brands are available - some local, some imported.
Apart from plenty of hiking, my Backpacker Boulder has also done excellent service on several overseas trips. A friend of mine modified his pack so he can padlock it, but with a few knots in the straps and putting discouraging things like dirty underwear in the outside pockets, I reckon you can discourage most luggage tamperers.
Final comment - young hikers should not go for a small pack "until they get bigger". Packs are so pricy that they will be stuck with a small pack which becomes increasingly frustrating after a year or two. Go for a "real" pack as soon as you can, or borrow until you can afford to buy one of your own.