Where about in the world are you based?

In the UK 7-8kw is less than most electric shower heaters now so it may need a new cable running but should be do-able (providing no one in the house takes a shower at the same time ;) )

There are second-hand options but rare, have you got a "watch" registered on the 'bay so it emails you if something pops up ??

Seems the spindle is always the worst / most expensive part of any build.