Why is it important to make sure that an existing Apache server is not enabled?
How can you specify the location to install Apache?
Only one server can be listening at the same IP and port combination at any given moment. Having previous versions of Apache running will prevent new ones from running and will likely confuse you.
You can use the --prefix option of the configure script. If an existing installation is present at that location, the configuration files will be preserved but the binaries will be replaced.