Depending on your skill-level and other requirements I would recommend these:
-
Amember
- A custom solution
These are two I would recommend for your requirements.
WordPress integrations tend to have a lot of features you don't really need and these bring a lot of security risks, potential incompatibilities (with themes, plugins or server config) or specific workflows.
Confirm your full solution and get it reviewed by someone that knows its stuff.
You could get:
- A security review
- Info about costs to design themes, update or upgrade
- An integration time estimate
- An Overview of potential incompatibilities/limitations and how to address them
I'm french too and from Quebec and I could do that for you (would have to confirm based on your launch date).
The last thing you want is a weak solution putting your server, your content or your sites at risk.