Hi Colleen,
You don't need to build the fields in Salesforce first, as long as you don't need this information to be available in Salesforce. If you're only going to use this information in Marketo, you can build the fields directly in your Marketo instance to prevent cluttering Salesforce.
Since you informaiton is chaning on a monthly basis, I would recommend logging a task each time a survey is completed with the survey results in the task comments field. This would allow you to always have a record of what the field was before it was overwritten by a new reponse.
Hope this helps!
- Ashley