Just pay 2x for the hardware and have a hot standby, 1990s-style. Practice switching between the boxes every month or so; should be imperceptible for the customers and a nearly non-event for the ops.
How many hours of labor does that take every month you failover? What about hot hard drive spares? Do you want networking redundancy? How about data backups? Second set of hot servers in another physical data center?
All of that costs money and time. You're probably better off using cloud hosting and focusing on your unique offering than having that expertise and coordination in house.
All of that costs money and time. You're probably better off using cloud hosting and focusing on your unique offering than having that expertise and coordination in house.