1. Depends sometimes on the style you are looking for. For most maps I think Gimp &/ Photoshop are good tools.
2. Doesnt good copyshop offer plotterprints of largescale Files? I am from germany and they do have such offers if you look around a bit. You life in NY? Try the Printed Matter shop there could be someone who can help.
3. Would be interesting what the veterans here on CG would answer.