How much domain expertise and network does a special issue guest editor need?
I've edited journals without holding a PhD in the past (don't ask). Doing a mediocre job is not that hard - to invite reviewers for example you just need to do a literature survey of the paper's field, which is standard Masters-level work. You can then invite reviewers even if you don't know them. The success rate is quite good - I think roughly 50% of the reviewers I invited responded to the invitation (although a substantial number decline). Doing just this, it's possible to keep the journal operating, although of course it will not do well. Since you know something about the field you should be able to do better: for example you can desk reject with more confidence, make sure every accepted paper has something interesting to say, and so on. If you can understand what the papers are saying on a high level, you should not have to worry about not being able to do a good job.

However, there are several potential problems I can see:


You've said it's an open access publisher based in Switzerland, which pretty much identifies which publisher it is. It doesn't have a great reputation. You could meet people who disapprove of you working with that publisher.
More concerningly, you are very likely to be asked to help solicit submissions. This is very different from reviewing submissions, and here a wide network will be helpful. You may also feel like the publisher is exploiting you - viewed one way, you are using your contacts to help them make money. If this troubles you, then that feeling could be exacerbated by the low response rate from other potential editors.
Yet another potential problem is "The title is broad enough to be its own journal". I would guess from this that the initiative for the special issue came from the publisher's staff, not the editor-in-chief. Although this isn't a red flag - there's a good chance the publisher got the editor-in-chief's approval before approaching you - it is something to keep in mind. Another issue is the broad scope could make it difficult for you to even put together a special issue since so many topics would be relevant. You could talk to the publisher and/or the editor-in-chief to narrow down the topic, if you feel it's appropriate.


One more thing probably worth mentioning is that you'll likely have a lot of scope to define what you want the special issue to look like. If the initiative really is from the publisher, chances are they will be willing to defer to your expertise, and the editor-in-chief won't object.

So, what to do now? If you don't care about doing a good job, you can go ahead. Leave the scope, invitations, marketing, etc to the publisher (they'll undoubtedly do a mediocre job at best, since they have no domain expertise) and just review any papers that actually are submitted. The special issue will not be pretty, but the amount of non-review work you need to do will be minimal. If you care about doing a good job, I'd suggest thinking it through before committing. Talk to the editor-in-chief, talk to a senior colleague, think about what you want the special issue to be about and who you might invite to contribute.

By the way: if you do go ahead, I'd recommend asking the publisher if they can offer free open access for your invited submissions.