All good thoughts and the basic wisdom is any bike can be used for touring. I used my 70's Peugeot PX-10 that I had muscled a $30 Planet Bike rack that I got at REI. Probably had some of the best tour riding I've ever had on that machine. Was out of the saddle a bit, but felt real strong with panniers on that bike. I've also toured on a dedicated tour, a Soma Saga. Touring bikes, dedicated ones like the Saga, are stout and designed to carry lots of weight for thousands of miles, over rough terrain in environments where reliability of the frameset is paramount. That said, dedicated tourers are heavy (how my Saga is soooo heavy with top quality parts is beyond me) and not the most responsive bikes around.

Touring bikes share common attributes, aside from their stout frames and significant capacity. Namely they can fit larger tires, 38mm+ is a general rule. Also, I would say that a dedicated tourer can handle a variety of road conditions with ease. I take my Saga on gravel and fire roads with panniers. Touring bikes will be more solid with a pretty significant weight in the panniers. A wide gear range is also a defining feature I would say. I recently switched from a triple with a 24t small ring (which i didn't really use) to a double with a 30t small ring. Of course I've got a 12-36t in the back. Quite a bit of range. Don't really want to be out of the saddle on that beast.

That said, as others have said too, is use what you've got. Buy a cheap rack and a set of panniers from REI, so you can return them if need be. See if you like bike camping / touring (we hope you do, who doesn't???) and then buy what you think is worth it for you. I wouldn't scrimp on a rack and like other have suggested, some of the better/popular equipment (ortlieb and tubus, come to mind) have pretty solid resale value. If you really want a good value in a bike get a complete bike and better yet a used bike that has been tuned up by a mechanic.

happy adventuring!