The basics you are asking for are pretty simple. You could probably take your generic bulletin board software and customize it to your own needs. You could download one of the many packages out there for free, install it on a Linux(also free) box running Apache (yeah, it's free too) and test your "product" in the convenience of your own home. By keeping everything free, you can only lose time and not money during the "research" phase of your business venture.
If you don't feel comfortable with Linux or Apache, you could put IIS on a Win 2k or higher box and test from there. Just make sure that if it is on a public network, that you have it patched up to current M$ "standards"
Or you could write something in Flash. You can make some pretty cool interfaces if you don't fall into the trap of going overboard with the gizmos and crazy abstract whatever-ish-ness that kill otherwise cool sites. Oh yeah, flash is costly, but with the right skills, you can some really cool things. At least you can download a demo of it, which can be opened later with a key, and learn how to use it enough to see if you want to continue it. It comes with a great tutorial and there are plenty of good online resources.
I guess at this point I'm wondering if you personally are looking to develop something or if you want to purchase an "engine" elsewhere.