Seems to be a difficult one :-)
I'm looking for an online solution where each of them can edit it's own page, editable fields would be great, as (WYSIWYG), so the only thing we have to do is export to a press pdf and send to the printers...
Do you mean something Web-based where you post the template and your clients each customize it online? Are you expecting one PDF per client, or one PDF that makes enough pages for everyone, when you are done?
I did tech support for a year for PrintUI.com, which offers this kind of functionality (on a one PDF per client customization, not 90 customizations makes one PDF), but it is a subscription service and not inexpensive, so you would want to be doing quite a bit of this to justify the cost. There are other services out there, too, though I don't know names or costs. Googling something like web-to-print might get you some more general info, if not specific companies.