What is a good bicycle for travel?
First thing should be the seat or saddle. I swear by “Brooks” leather saddles. Its like a hammock for your arse. Standard foam seats eventually compress and you are sitting on the plastic base. Even the gel ones eventually let you down.
I chose my touring bike because it has a very long frame. It is a Surly -Long Haul Trucker. A long frame means the top tube that runs horizontal between your legs is longer than usual and so is the chainstay (the horizontal part of the frame near the chain). A long frame means the bike is more steady to ride if you are carying a load.
If you are riding mainly on the tarmac and in the developed world, than 700C or road bike wheels are my choice. They roll over bumps smoother and offer less rolling resistance than mountain bike tyres or 26inch rims. They are also more flexible so they soak up the bumps. 26inch wheels are easier to have repaired by any bike shop in the world and handle off the road better.
Panniers or trailer? I have never riden with one of those little bike trailers. If you are riding alone with a trailer i would imagine it would be very difficult to pick up your bike and walk up a flight of stairs. It is surprising how often i found myself doing this. I actually find my bike with full panniers to be more comfortable to ride than without bags. The weight of the bags stabilizes the steering and makes the frame soak up bumps. If you had an alloy framed bike then panniers might not be a good idea as the added weight on the frame might cause a break. Buy a steel framed bike!
Hope these tips are helpful.
How to travel by bicycle cheaply in Europe