My advise would be to not create surveys with Marketo. It will completely mess up your custom fields. You'd need new custom fields for every question. And those fields you can only use for that one time survey. After that you'll need Marketo support to remove the fields. Another reason is that Forms 1.0 has no way to alter questions, based on what was previously provided. Forms 2.0 will allow you this (hopefully in this month's release). Just use SurveyMonkey or another tool, but use Marketo to send the invite.
A great idea would be for Marketo to integrate SurveyMonkey results (e.g. completed survey) so you can track conversion. I have created an idea for this: https://community.marketo.com/MarketoIdeaDetail?id=08750000000JYFaAAO