Is the RETEX-24 enough for a whole house?

It is a whole-home electric tankless unit, but actual capacity depends on your incoming water temperature and how many fixtures run at once. In cold climates it may only handle one or two uses simultaneously.

What electrical setup does it need?

It is an electric tankless heater that draws high amperage, so it needs dedicated circuits sized to the unit. Most homes need a licensed electrician to confirm the panel can support it.

Does it work with a standard tank replacement?

It mounts on a wall and connects to the same hot and cold plumbing lines, so it can replace a tank heater where space and wiring allow. The electrical side is the main difference from a tank swap.

How does it compare with a gas tankless heater?

Electric tankless units avoid gas venting and can be easier to place, but they draw heavy electrical current. Gas tankless models usually deliver higher flow rates for whole-home use.