hello…
open source vs. paid is pretty much personal preference. many open source projects these days are extremely well documented and actually have staff that are more responsive than a paid product – it just depends if you get one that is like that. best way to check is to read through the docs and see if they have a forum with active and recent posts – a lot of activity shows that a product is well maintained and supported – also skim them for general tone of posts to get a feel for how well the product works. if you do that (and in this case, bulletin board software is a very common open source project) then you’ll be better off not paying for what you don’t need to…
a very common open source bb project is phpbb – give that one a shot. or if you see one online, it’ll likely have a “powered by” or something similar with a product name – if you like it there, visit their site and see more about it…
good luck!
sitruc