Just share of min opinion.
I had open the panasonic(U have to open the cover when u are going to fix it at wall). The water pipe include heater tanker are made by copper.
Some brand are made by plastic.(electrolux economic model is. I was purchase and go exchange to Panasonic next day).
From the internal parts quality, I only choose panasonic.

But I do not have chance to open Joven and Alpha caseing, no command on this 2 brand........

If u afraid water temperature not high, choose 3.6kw. Now most of the heater are 3.6KW
If u prefer water with build in pump, please choose new model with "DC pump" it is much quiet than old type AC pump. But I am not sure anout water pressure is satisfy for u? Coz my old panasonic with AC pump reli not big pressure.
After fix to direct water I found water pressure is stronger when I disable the pump.........quite strange.
I was used a old panasonic with pump....reli noise. Now I fix a groundfos pump and disemantle the pump wire at my panasonic heater ^^

If u plan to install more than 3 water heater with pump, why not consider to install water heater w/o pump and install a pump after your tank to support all water heater. It might slligtly expensive, but wroth it loh....

Just my personal opinion