1. know your bb to seat dimension. That way when you go to test any bike you set it up the same every time.
2. know your prefered handlebar position when you get on a bike. This is something you can eyeball measuring with elbow at the seat and seeing where your fingertips go in relation to the bars. It's just a gross measurment but it'll tell you whether you need a new stem.
3. an old used "road bike" won't be as adaptable for touring as an old used mtn. bike. regarding weight carrying and rear wheel strength. That's a gross generality that can be ignored with specific bikes.
4. you haven't said whether your $400-$500 budget is bike only or the whole deal. If it's bike only there are new bikes that can do the job, if it's the whole deal you should continue what you're doing, gathering information, and looking for a free bike where you may only need to replace the rear wheel, cassette and chain.

Just looked online at the bikes sold at the Bike Loft, a Marin CS1 for $429 could do it