I feel that I am an expert on portable classrooms!
The school I taught at had 30, yes, THIRTY, portable classrooms. Grades 1-3 were in portables. The one I taught in was probably @ 15 years old, and was really pretty nice. Definitely no problems with mold. Most teachers liked them better than being in the building.
Pros:
Quiet and few disruptions - in the school, there are always people walking by your room, looking in, etc.
You can do boisterous activities without bothering others
In our portables, you could staple in the walls so you could put stuff up all over the room - in the classroom, only on your 1-2 bulletin boards.
Climate control - you can adjust your own thermostat. In the building, you are hot or cold and just have to live with it.
Cons:
Walking to and from the building for bathroom breaks and for art, music, PE, etc. in inclement weather. Our walkways were covered, but when it was rainy and windy, we got wet.
Distance from the main office and nurses office - if you have an emergency, it takes longer to get help.
Security - I worked in a high crime area, and our portables were broken in to frequently and electronics stolen out of them.
Security - you always need to send students in pairs when they go in to the building without the whole class.
I do NOT think you need to stay up at night worrying over this. The idea of your 7 year old walking through a snow storm to go potty isnt a great idea, but its not the end of the world either. I grew up in Maine and we had recess unless it was colder than ten degrees out.
I would worry MORE about what teacher she has than what classroom she has. However, I do think there are very legitimate concerns for a parent of a 7 year old and you have a good reason to ask to move her if you do not feel comfortable.