You could create a new group, where new users would be added to that group. In there you'd set the maximum private topics (under site permissions) to 0. Then under group management you'd add a 30 under the Post Count field to the regular Users so users would be added to that group when they've made 30 posts. You'd set the maximum private topics and messages, so they could then send private messages.
No way to prevent people from posting for 2 days.
Private message control is under Site Permissions now and is group based.
