An automated generator system will probably cost more than $2000; probably $3000 with professional installation. The Generac 7 kW versions are available at discount in the $2500 range and can be run on propane or NGas.
You could set it up so it senses utility failure or flooding and just operates the pump. It will be more if you want it to automatically transfer the pump from utility power.
You need enough fuel to run for the unattended period.
If someone will be around, you could increase battery backup time with more batteries and use a manually started generator system for a lot less money.