A cloud environment might work for you, especially if very brief downtimes at odd hours are ok when you need to scale up. How you construct it will depend on how hard the server will need to get hit at maximum capacity.
One possibility would be to start with two servers (one for the eZ Publish installation and one for the database), both cloud instances, both relatively small. As the load picks up, each could be increased to a larger size (which may require a late-night reboot, depending on the provider). That would give you several steps of scalability, without having to make any changes to the files. Or you could start with three servers small cloud servers (the two mentioned already, plus one which handles the admin interface, if its traffic is substantial or if accessing it in peak times could become a problem).
