Things to conside: If your chainstays are too short, your heels may hit your panniers. Gearing should be low enough for long climbs on steep hills. Wheels need to tough enough to stay true when riding on rough roads with a big load.
Long Haul Truckers have all of these characteristics and are comfortable in the saddle for all day riding. Vintage mountain bikes have similar characteristics, but usually need bar ends or something for more hand positions.
Lots of people tour on lots of bikes. The most important feature of any bike is it needs to fit you.