So is it possible to have a single server set up right now and after some time extend the set up to multiple servers?
It is possible to extend a single srvers etup to a multiserver setup by adding additional slave servers to the master server.
But I would recommend something different. Install openvz on your first server (just the openvz kernel and tools), then create a virtual machine for webserver, mailserver and one or 2 dns servers on the shell. Thats quie easy and we have guides for that here at howtoforge. Now install a multiserver setup inside these virtual machines. So you have 1 physical server that runs 3-4 virtual machines.
The benefits are:
1) Security: web and mailserver are separated.
2) Scalability and maintenance: If the load on your server gets too high or you run out of disk space, you can simply move the vm with the mail server to a different host. And you want to use new server(s) in a few yers, you dont have to reinstall the machines, just install openvz on the new server and then copy over the vm and start it there.
3) backup: With vzdump, you can easily create backups of the vm's