A water powered backup is impractical. It would take more than 1000 gallons per hour from the city water system to provide 1/2 HP at reasonable efficiencies. And you would have to get rid of that water too, as well as paying for it and in most cities paying to put it into the sewer.
There are lots of possibilities for backup:
1. Your battery system for short time backup
2. Automatic start generator
3. Battery powered alarm that wakes you at night or calls your cell phone and/or a neighbor so you can get someone to go in and start your generator
4. Arrangements with neighbors to start the generator if power goes off and you are not home
When you have a backup system you should test it once a week to make sure it's working. The Generac system that I am familiar with does an automatic self-test every week.
I usually apply my engineer mentality to these things. I would:
1. Put in a duplex pump system, which is a system that alternates pumps to keep them operating.
2. Generator with alarm system; probably not the Generac because of expense; but an electric start generator. But if you expect to collect the cost from the seller then go with the auto-start auto-transfer system like the Generac, including an automatic transfer switch. (Things are different if you are spending another person's money.)
3. An alarm system that notifies you and a backup person by phone if the water rises above the maximum control level, indicating a problem that must be dealt with.