Re: Forum with 15k users, 100 u. online in one moment
There are already sites running bbPress 0.9 with many more users than that.
Only thing I would suggest as I have in the past is that if it’s a commercial project with income, you should always be considering vbulletin which is no-nonsense, high end features/performance right out of the box. But it’s not cheap and everything worthwhile for it is usually pay-only also.
bbPress is for do-it-yourself-ers who want to make something that either blends into an existing design or you want something fresh that doesn’t look like every other cookie-cutter forum.