The BBS used by PPRuNe was written by infopop. Go to their website
www.infopop.com and you can get UBB-lite for free.
Also look at
www.vBulletin.com as they also have a lite version. Their full version is apparently very good, and a lot cheaper than infopop's current prices.
Then when your site gets popular, find a dozen friends to help you run it and make sure you have a well-paid job to finance it
---PPD