[Sim9]

Three options I see for you:

1. Set up two usergroups, one with PM permissions for trusted users, one without. Then have an automatic promotion at X posts between groups.
2. I just now uploaded a PM limiter. You could change the configuration values to limit users to zero PMs per any time period until they have X posts.
3. There may be another plugin solution -- but I didn't see any in my quick search before writing #2.

Hope that helps;