It's probably more recommended to make an announcement thread and start a signature campaign , that could generate much more traffic for you+ more users.
I disagree with this. The likely outcome: Campaign gets started -> Improperly managed -> Filled with spammers -> Service ignored by any major users.
OP the best option are those AD slots, and the price fluctuates so you may end up getting it a bit cheaper than usual. The secondary option is a signature campaign as suggested, however make sure you have a
very good campaign manager.