that stainless jobbie looks the go..
is that 2C+E orange circ (as in underground, it looks stiff and not like flex) ) or flex, what mm is it? how did you work out what cable to get?
as far as the pic will let me zoom in and still make out the pic i would say its 7 strand and as you alluded not suitable for flexable cable.
there are small stainless boxes/enclosures available
the gland housings in the earlier posts linked by unwritenlaw and nosco look nice and would be fine.
there is gel available that is used for wet areas and to fill these boxe$$$
plenty of options here, good old 5 min araldite is prolly the easiest most commonly available, scotchcasts which are used in industry use an epoxy resin, another alternative is mastic but its messy and hard to get where you want it without burning everything.
^ all things a sparkie will know about...there wouldn't be too many sparkies out there that haven't had to change a hot water element...you might not need an appliance technician
if a sparky had never changed a heating element he would have had a very limited scope of work,
Mate that'd be amazing if you could source one. I need to double-check the diameter of that hole... think it's 1 1/4 inch, but have to double check.
yes you would need to be sure of the thread size on the element to know for sure, alternatively if you knew the size you could couple some basic bits together and acheive a similar outcome, jusr remember that the element is quite possibly a pipe thread and the gland entry will be m16 or m20 thread.
edit, just looking at the first picture , it may be a pain in the ass to earth that element, is will then rely on the sleeve contact with it which will need to be solidly earthed.
edit ...edit the last pic from this link shows the earthing arrangement,
https://store.brewpi.com/mashing/stainless-steel-heating-elements/heating-element-enclosure-kit-178