I got a Highlander NI Clone (Olive Green) earlier in the year. Very good for hill walking and holds all my gold panning equipment with ease. To tell the true I have only used it twice for TA stuff!
There is no point in spending loads'sa'money on some thing you will use once in a blue moon.
On the Camelbak note, I suggest the BFM 500. Tons of space, relatively affordable, quite comfortable with body armour, a lot of good smaller pouches, and I found that the quality of construction can definitely withstand Afghan wear and tear - quite a pounding, in fact. Comes with a hydration reservoir, so money saved.
Add a pouch like this one to your body armour - you can transfer your camelbak bladder to it when you're not taking the daypack, and you don't notice it when it's empty.
PS: If you have more money, look out for some of the new packs specifically designed to be worn over body armour.