I would like a forum based website where as people sign up to "answer questions", people post questions for free and when they get the correct response "they pay how much they feel the answer was worth".
Also for future visitors to the site; in order to see the answer to the question they need to have a subscription.
Its pretty simple. Whichever open source software you feel is best.
PayPal Payments Pro will be the gateway.
Please note the subject of the forum will be purely Magento questions - nothing else.