I hit up the Des Moines Craigslist and found:

http://desmoines.craigslist.org/bik/1175843248.html

http://desmoines.craigslist.org/bik/1175426594.html

http://desmoines.craigslist.org/bik/1174467622.html

http://desmoines.craigslist.org/bik/1172683264.html

These are all relatively inexpensive, and would work for lightweight touring. Flat bars (like on the mountain bikes) give some people wrist trouble on long rides. Bar ends run about $15 or $20 and can minimize that nicely. Bikes like these may have short chainstays, but your Walmart choice likely has the same problem. The shorter the chainstays, the lighter the load needs to be... which isn't a bad thing in this case, since lighter loads mean you can get away with a cheaper rack and panniers. The more you carry, the more the bike needs to be designed for carrying weight, and this adds to the cost.

Poking around at bike shops that sell used bikes is also worthwhile. You'd pay a bit more than at Walmart or on Craigslist, but you can usually find a bike that's *meant* for carrying a load too. And the bike shop will make sure the bike is in proper working order, so you won't run into any surprises with your brakes or shifting.

If you really need the bike to be as cheap as possible, for as long as possible, I'd hunt for a local bike co-op. Bike co-ops teach bike repair, and have build-a-bike programs (sometimes only for kids, sometimes for everyone) where you can earn a bike by repairing it. Since fixing a bike can get pretty expensive, this is a good route if you need to save money. The more you know about bike repair, the easier it is to do things on the cheap.