For new members, I'd like to prohibit them from posting links until they have at least (variable number) posts.  This will help discourage spammers. 

I thought IPS had that feature a few years ago, but it seems to have been dropped.  Please bring it back.

Yep, I'm fully aware of the other anti-spam features in IPS, and I use them.  But there's nothing at all like this.  Think of it as a "backstop" in case a spammer slips by the other features, which seems to be happening with increasing regularity, despite changing the QA response often.

+1 on this...currently its either moderate all links or nothing. I'd like to moderate links for members with less that x amount of approved content items or something like that.

It may help:

 

I think it’s better to moderate the content instead of disallow because the user can remove the http(s) and www and simply post the domain.com.

I also have a plugin here that can do something similar:

Adriano's application is more advanced / has more features, mine's just for more brarebones link filtering.