I'm interested in how much money do I have to invest if I want to buy Bitcoins? Is there any minimum?
On the whole, there is no minimum investment for Bitcoin, as it is not a security such as an ETF or mutual fund purchased through a brokerage or management firm.
What minimum purchases exist on Bitcoin exchanges are merely requirements set by each individual exchange to reflect the minimum purchase/sale that they feel is necessary to cover their overhead for each transaction.
Purchasing from an individual trader, you'll find that there is generally no such minimum, aside from whatever minimum they set to make it worth the time and travel expense for a rendezvous.