Well, you are probably talking about a used bike. There are a few ways to go. If you get a trailer you can put it on just about any bike you are comfortable with as long as you have a good rear wheel (32 or more spokes). If you are planning to get panniers then you'll need to look for a good used touring bike.

Read the other threads here to see the bikes people are talking about. The usual suspects are the Surly Long Haul Trucker, Trek 520, Jamis Aurora, Cannondale T1000/T2000/T800/T2/T1, and a bunch more. If you see a touring bike for sale just come back here and ask people about that model before you buy or bid on it. You'll generally get good advice about anything you'll find.

Nut as I said, if you go the trailer route then just get a bike you are comfortable riding long distances. Just don't get anything too exotic or with wacky components. Steel frames and Mid-range Shimano components seem to be the best choices. Mountain bike gearing will be better than road bike gearing for the most part but you can lower the gear ratio on almost any bike.