I use a 30l burco boiler for my sparge water.
- What holds much holds little. So if you don't need 30l, don't heat 30l
- I heat extra and use it for cleaning as I go
- I can use the kettle to catch the run-off water from the cooler, again used for cleaning
- I can make tea for a lot of people if I need to
The one you linked has a sightglass which I wish I had (also wishing I had one on the Grainfather). However, looks like that tap might be hard to replace if you wanted to (I replaced mine with a proper ball valve tap).
Additional consideration is just making sure you have enough current available on the circuit to have both the brewzilla boiking (1900w) and the sparge water heater heating (look like the one you linked is 1300w). That's more than a 13A draw there, so would need to be careful