Surplus stuff is good but remember, it was built for a specific purpose and comfort was NOT usually a top priority.

If you stick with some of the brand names you will know what you are getting yourself into. I own a Gregory Palisades (very comfortable, very durable) and I use an older Marmot Talus every day. Both make great packs of all volumes.

Internal vs External is a great argument but so is Pickup Truck or passenger car.

External Frame: Greater Load capacity, able to strap directly to frame, easier to repair, usually much cheaper

Internal Frame: Usually more comfortable, much better load control on technical moves (off trail, climbing, skiing, etc), easier to get around with (no frame hanging off to snag).

Definately take a look at what you are going to use it for and Try It On! Any good shop will fit you and allow you to "load" the pack with their sandbag to test suspension.

Enjoy.