We have a total electric home and want to replace the old water heater with an electric tankless whole house hot water heater. Gas is not available where we are. Is it practical?

It can be, but you need to determine the load of the tankless since they use a lot of immediate need power and it is possible you may need to upgrade your service. Personally, I am not a fan of them, particularly if you have a larger family using water at one time, they won't keep up with demand. I would consider a newer, more efficient tank heater with an extra insulating blanket.

Not a big fan of the hot water on demand system. Did You know that if you go to wash your hands or shave your face you have to move X amount of water for the system to kick in and start making hot water. Take a look at the marathon water heater.