As a general piece of advice. Your comfort is far more important than the size of the pack. You'll be carrying alot of weight. Have you been a guide before? You'll see that One Planet packs are the most common brand used by Outdoor instructors.
I personally have a Lost World, but I bought it in 2005. I purchased it for the same reasons you suggested. I was a guide, I wanted something large, durable and comfortable. In terms of comfort it was a very close competition between the W.E. and One Planet. So I ultimately bought the Lost World as I felt shape of the sack was superior.
You really must try and carry the largest One Planet, W.E, Mont, Macpac and Aarn.
In terms of buying an Aarn pack. If you don't want to use the front pockets then you don't need to. But you will be limited to 77L
http://www.aarnpacks.com/#!load-limo/ci5c with practice you'll get used to packing into the front pockets.
But More importantly though, if you happen to like another brand such as One Planet then you should consider adding Aarn front packs to it. You'll gain more space and better balance. It's much easier to do than people think.
In addition to this, another common way to add more space to your pack is to attach on to the compression straps.
I've written a few times on this forum about large packs for guides.
There are other packs you could consider, but I feel trying them on is absolutely crucial, and so you are likely to be limited to the brands you can find in shops.
But since you asked.. here are some other options
http://www.cactusoutdoor.co.nz/packs-bags/tramp-walk/foray.htmlhttp://www.summitgear.com.au/summit-gear-sabre-rucksack/None of those packs are really large enough, but you could add front and side pockets. There just aren't many brands of packs that make them as large as you want.