You should be able to get a dedicated IP address from your ISP, or one of their competitors if your ISP doesn't provide them. You can use the name servers provided by your domain registration service to provide name resolution for the dedicated IP address.
To reduce the risk of compromise, you should use the firewall to close access to all ports that are not running services and restrict access to critical ports such as port 22 for SSH and port 5900 for remote desktop to only the IP address blocks you connect from. Read through your log files daily and block access to IP addresses running port scans, unauthorized login attempts, injection attacks, etc.
For Apache web server security, the Apache security module (modsecurity) is a must, but Apple doesn't provide support for it and installing it on OS X Server is a complicated process. It's an inexcusable, irresponsible lapse in security on Apple's part. Running PHP web applications on Apache without modsecurity represents a very high risk of having your web site compromised at some point.