If your intention is to add chat to your existing site, that's a great idea. Your users will return more frequently, and may end up using the chat more than any other feature on your site. Since there is very little difference between adding chat and starting a chat community I won't bother to distinguish between the two.
The first thing to consider, and this will make you or break you, is the name of your site. Even if you're adding it to your current site, a name is needed. It will help theme your community and make adding rooms more logical. Names with "chat" in them seem to work best. It's easy to feed the old ego and pick something like "Mike's chat place" but in the long run that's a dead end with a bleak future. "Chatropolis (SM)", a word I coined after a few drinks, has many more possibilities. It creates images of a city and has possibilities for endless rooms. It has worked incredibly well for us.
Your next decision will be server software. Unless you plan on staying small and crashing a lot I would stay away from Windows based servers. A UNIX based server will give you a lot more bang for your buck. They range from Free to 100K. We even sell the server we run Chatropolis on. Buy a server that does not have restrictions or is priced on the amount of users online. This is generally a bad idea for everyone. Do your homework and demo all servers extensively. Before you buy anything send a fake support e-mail to see how fast they answer. This is very important if you think you will need a lot of support.
Policies & People
You'll be writing a lot of policy for the users and changing it often until you get it right. One policy I am highly against is making a user register. This stops growth and will kill a good site in a few weeks. You want to get users in the rooms and keep them there. I suggest that you make your site free for at least the first year and always have a nice free level of access.
The key to having a fun site is new people. You can never have enough new people. If you run an adult content site, make the chat available to everyone, members and non-members alike. This will bring people back, and every time they return you get a shot at selling them your content site.
Should you charge? Well if you are strictly running a community site, then you will probably have to. If you do have a level of paid access, make it just that. Always have a lot of free spaces available. You will never get someone to pay for an empty site. Add special features for members so that membership is a value. If you run an adult content site you may want to include special chat access in a membership for your main site. But remember if your main biz is content you want to get the users back. The chat won't make you rich anyway.
One of the biggest mistakes I see chat masters make is in contact information. If you want to build trust that leads to a purchase, you won't do it with sketchy contact info. On our join form I include the number to the telephone on my desk. We include our physical address, mailing addresses, main telephone number, D&B number, corporate info, plus my name and the full names of some of my staff... If you're afraid to publish this info, how can you expect a user to trust you enough to pull out their credit card?
You must have a great support system. All servers have quirks that may require some users to seek help. Have an extensive FAQ, knowledge base and support software that allows you to use templates (you will be answering the same questions a lot). Update your main page with system news often – users will read it and find it comforting to know there really is a human behind all the HTML.
Why do sites fail? Mostly it's from unreasonable expectations of the creator. A chat site takes a long time to become popular even with extensive advertising. They are difficult to build up but the flip side is that once they get going the users are extremely loyal.
Michael Ludwick runs Chatropolis, one of the largest and most successful chat operations.