Which server?

I’ve been building Web sites for non-profit organizations for several years now, and need a leg up as far as trying to become a freelance Web Site Creator for Merchant accounts now as well. The problem is that every potential client wants me to host the sites as well, and since I’m just beginning, I don’t have the hardware to support it, nor do I know if I really need it. I have encountered many opinions regarding servers, including the one that I most likely will require a custom built serverto house numerous ISP’s. I need some input as to the facts.