User Registration: allowing users to edit their survey?

- provide a multi-page survey/questionnaire to my users, when they are logged in (checked!)

You can certainly restrict your forms to logged in users. The quiz will take some work on your part but is achievable based on what I know so far.

- when the survey is filled up, they can order a product, and select a certain quantity of items they would like to get (i think this is checked as well, right?)

You can create order forms. I'm not sure how the survey and the order form tie together, but if you want to quiz someone to determine their needs, then provide product suggestions to them, you can do this by passing the results from the quiz (form one) to the order form (form two.) Then, with conditional logic, you can display suggested products based on the results of their quiz.

- when the order is sent, to send an e-mail to the user AND to the admin or some other person (I believe this one is checked as well)

Included

- the user should be able to log-in and edit his survey and answers at any time (the survey is really long, so he should be able to quit/save and come back at any time) => this is what I'm really unsure if I can do?

Not currently possible. There is no built in functionality to allow a logged in user to come back and edit their entry. There are some 3rd party solutions out there which purport to do this, but I have no first hand experience with them and can't recommend any of them. And it's a commonly requested feature, so I suspect it's being worked on for a future release.

The 3rd party solutions are not linked to Gravity Forms at all. I've seen a few posts but very few actual solutions. Most of them focus on the user editing their post, not their entry, as well. Here is one such example:

http://wordpress.org/extend/plugins/gravity-forms-update-post/

That updates the post, not the entry, which is what I think you're looking for. I can't find any examples of entry editing at the moment.

You can find some 3rd party solutions listed here:
http://www.gravityhelp.com/other-resources/

I don't see any that allow the visitor to edit their entry.

Thinking about this though, you could always pre-populate and edit form with existing values (so, a nearly identical form, but instead of blank values, the form fields would be filled in with values read from the previous entry.) That part is not difficult but I am not sure how to secure access to the form and ensure it's only editable by the correct user.

Sure. Hope you can make something work.