If you are considering paying for forum posts then growing your forum is perhaps a direction to move in. I have never paid for posts and wouldn't ever consider it. I just grow the content side then place links from there to the forum, pay daily attention to the forum, cultivate repeat visitors, treat the forum like a community, encourage questions from the contact and about us page to be placed in the forum. Above all, research your competition, make their weaknesses your strengths. You don't need to go head to head against their strengths at this point. The goal is to cultivate community growth by offering something unavailable elsewhere by creating a valid proposition for why someone would want to switch from brand A to B. Which is where researching weaknesses, the things that cause dissatisfaction comes in. Those dissatisfactions and weaknesses can form the backbone for growing your community.
Advertising on AdWords is an option, as is advertising on other non-forum sites. Once the community has reached a decent momentum then you can start the link building. The link building for forum begins with the link beg campaign, the old research the competition approach. That's the starting point, but after the community has momentum.