If you are towing a trailer, why not use whatever bike you already have, and then after a little while on the road, you'll know a lot more about what you want. I found that flat bars didn't work for me, and that I didn't like trailering, I like panniers lots more. But if you already have a bike and a trailer, just go!

If you don't already own a trailer (warning, i'm biased) don't buy one, just go straight for the drop-bar-style touring bike that fits you best and get racks & panniers.