Greetings Regina , Welcome to DotNetNuke!
1. Installing DotNetNuke won't have any direct effect on existing websites. Adding any website to a server may cause some additional traffic, etc. but there is nothing specific about DotNetNuke to consider there.
2. DotNetNuke should run fine on your IIS-7 configuration. We tend to keep the requirements minimal so that it can run in almost any hosters environment without changes.
3. DotNetNuke won't make any changes to IIS at all. It will install a web.config file for itself in the site root directory, as do all IIS applications. But this has no effect on any application outside of the DotNetNuke install.
4. Not that I know of. Does this server have any special configuration you can identify that you want to ask about specifically?
5. As I mentioned, there should be no gotchas specific to DotNetNuke. If you're planning to run DotNetNuke using IIS Express, then the web server would need that installed. However, if you're connecting to another SQL database all you'll need is the connection information during the install process.
Hope that helps.