I'd have thought any decent pack in the size range you mention would do the job, though something with side-zip access to the main body compartment would make your life much easier than dragging things in and out of the top.
I wouldn't normally suggest a travel pack for real outdoor use, most are bastardised crosses between a suitcase and a rucksac, but there are a few out there which are closer to a pack than a case. I'm thinking particularly of the MacPac range, stuff like the Zambezi (from memory) which is pack-shaped and has a decent harness system which means it carries like a rucksac, but has a full zip-panel opening system. Lowe Alpine also do something similar.
Might be a little over your price range, but more sensible I think than constantly delving into the top opening of a conventional pack. Avoid the big, heavy suitcase-type numbers though.
Met a guy walking inot Everest who was carrying a LowePro stuffed with camera gear, weighed about what you're talking. He got his wife and a yak to carry the rest of his stuff. Unfortunately yaks don't function at altitudes under about 4000 metres I think, so that's not an option...