Try 'bulletin board' in Google! There are free systems, but yes, your 'customers' WILL need to buy the server space (unless Danny is having a warm fluffy moment
) and you will need more than the 'basic' web server because the board will probably need to run PHP or similar and the access loads could 'upset' one of the low-cost providers. Once set up the simple ones run pretty well.
I know 'Keef' is involved in one of these and will, no doubt, be along to offer help - and there are others here. The size of your task grows with the requirements of the board!