I worked for a wilderness program w/ troubled teen for many many years (Wilderness EMT, Equipment Manager there as well), after Army and college. I humped various packs for years, mountain altitude to canoe portages, Canada, Mexico, and US. For BOB, I would consider external frame, well adjusted. They offer solid support and some degree of flexibility to configure and "odd" load if needed (strip off the nylon pack from the frame). You can also easily replace a ill fittiing or worn out part/strap if needed. Also, could retro fit a Mil Spec belt or other "system" type item to carry accessories within easy reach (think ammo pouch). Additional benefits are they can be found dirt cheap as you have found out. I have many around my place, one REALLY old Paul Petzolt (NOLS) pack from 1960's. Some externals are plastic framed, others aluminum. I found the plactic to flex just a little that was a POSITIVE attribute. Weight distrobustion within the external pack is even MORE critical than with an internal frame as it is further away from your body to start. Put weight high and close to your body, lighter stuff low and away. Worth mentioning is also the fact that in a BOB situation it might be difficult to keep the pack out of the weather, and I dont know that ANYONE has ever designed one, but a "Bill's Bag" (envision an Army duffle but bigger in rubber) rubber sealable pack like a portage pack (although they lack a frame or support of any type)would be the cats meow. You will likely not have control over exposure to weather conditions. You can get soaked, but you ABSOLUTELY need to keep your gear dry. It is nearly impossible without something like this:


http://www.scouttech.com/scripts/prodView.asp?idproduct=1634


Another advantage is the clean lines and lack of "stuff" sticking out that will catch on things, whether the environ is wilderness OR urban. Also, on an external frame pack, repairing ripped or broken strap is pretty simple. On a sewn internal, NEARLY impossible in the field (my teens could break ANYTHING and did)


Good luck, Ironwood