>> our premises are just around the corner from one of the biggest Apple Stores in the world, we can replace any of our servers literally within minutes if they fail
I sure hope you don't have a server failure outside of business hours :-)
I think we're prepared for that too. First, we have a few currently unused previous generation Mac Minis lying around that could be used temporarily. Second, since everything we need (Apache and PostgreSQL, minus Mono) comes preinstalled with OS X Server, these machines can easily take over each other's tasks temporarily, e.g. the database server can become a web server too until we can get a replacement. It's not ideal but it'd do during off-peak hours. Fortunately, the Apple Store is open even on Sundays so even in the worst case scenario we could get a replacement within 12 hours max, which is not necessarily the case with, say for example, Dell servers.
I sure hope you don't have a server failure outside of business hours :-)