The cheapest is easiest way is to keep main pages on a public webhost, and check eachother for server uptime and redirect as required.

For power, keep a...

• GenTrax Inverter Generator 3.5KW Max 3.0KW Rated Pure Sine Wave

Providing a power supply if power goes out.

• Internet connection

Keep your main pages on a public webhost, and check eachother for server uptime and redirect as required.

For drives: RAID 6, needs 4 drives minimum.

For other parts keep replacement parts and swap out as required.