Paul,

Electric water heater replacement elements are relatively cheap.  Each is rated at 5kW, 220VAV, if I remember correctly.  You can just hang them in a tub of water with an float refill valve to replace boil-off.  Check out the old style toilet fill valves. The hot parts need to be kept completely covered with water when ever powered.
Every now and then you will need to wire brush off any accumulation of of solids precipitated on the element.  I wouldn't expect very much, since they tend to shed the hard coating each time they thermal cycle.
You will need to "muck out" the tub as solids accumulate on the bottom.  Again, not very often.

Craig Taylor