In my opinion best boiler is massive cooper outside and very thin ( 1/2 or 1/4 mm sheet or metal foil ) stainless steel lined inside as a internal protective layer without great impact to thermal transfer , because heat transmission resistance of steel is proportional with thickness of the element x...

Gaggia Pure and Gaggia Color do have a SS boiler

No thin steel at all - is heavy - default pressure is 5-6 bar measured with gauge . Is more thicker and resistant that stainless steel boiler used in Tschibo Cafissimo . The steam exit tube cord is more fragile and it has no problem , or the ptfe tube between pump and boiler in

Steam generators for ironing like Rowenta DG 8760 has a large, beautiful (0,8 liters and 1350 watt ) stainless steel boiler with solenoid release valve and CEME 15 bars pump inside . It heat steam after one minute - is faster than many espresso boilers . I found mine refurbished at 50 euros . Can be...

Instead of zink - i recommend magnesium anode for aluminium boilers . Small pure aluminium anode i found on ebay uk verry cheap