Ok. As long as you do not run more then one web, mail, dns or database server, then its ok of course. But I really dont understand where your problem is then. Above you said that you want to run more webservers "4. one or more webservers" and then you need more IP addresses.
Are you really sure that it makes sense for you to run a multiserver system on a single IP dsl line? A single server with a current quadcaore and a few GB RAM is able to fill a 100mbit line already, so to make sense for a multiserver system, you would need a few hundred Mbit internet connection.