Excellent points. I also don't agree that it's necessary to save pack selection until last. Any competent clerk should be able to help his customer determine what their general approach will be: "I want all the luxuries of home" will lead him toward 70+ liter packs; "I'd like a nice combination of durability, comfort, and not break the bank" will move you toward 50-60 liter packs, and "I've read about lightweight, and I'm aware it's more expensive, but that's how I want to go" will put you into smaller-then-50 liter packs. Pack capacity will then force gear selection accordingly. I've done that myself, the last couple of times. If the customer says, "I want to backpack, and know nothing about it," then the clerk should spend some time educating the customer (and if necessary, guiding them toward a style. It may take a couple of visits before the buying begins, but a good store will work this way.)

I did a couple of calculations using your estimates, and they aren't far off my experience (if I base the pack size on volume used, not volume available.)

Send me your email in a PM; I've jotted down some stuff I give to newbies who want to buy a complete set of gear without doing research, and I'll send it to you. It's an off-the-shelf combination that I found works well, and is my current second set of gear.