In practice, most heating systems have substandard water and even my own heating system, after I powerflushed it, only JUST passed the most basic water quality standard someone installing a new boiler would require (and there's still a number of lazy installers - or installers willing to cut corners - who fit boilers without checking the water quality in the first place).
The fact that it was black sludge and not brown suggests that there isn't an awful lot of active corrosion going on - the sludge may have been there for years. Bear in mind that what could damage your boiler will be in circulation. The sludge sitting in your radiators won't harm anything. In fairness, a powerflush doesn't get everything anyway: I have personally (so I know it was done to a reasonably high standard) powerflushed the same system 2 years in a row and still got quite a lot out of it the second time round. And, if it's a sealed system (which I think is what you have), the corrosion process will be very slow anyway, unless you have leaks and are forever topping up the pressure.
Inhibitor isn't expensive, however, and there's absolutely no reason you can't add some, even with poor water quality.
If a powerflush isn't affordable, if you can find a plumber that will add a system filter (Magnaclean, TF1, or similar) on the boiler return (ideally without draining down to avoid bringing stagnant sludge into circulation - find a plumber with a pipe freezing machine?), that will do an awful lot to filter out sludge and prevent it getting into the boiler. If you already have a filter and said filter has been cleaned in the last year, happy days.
Also, I think your insurance provider has tried to brainwash you by using fear tactics. Yes, that's a reasonably modern boiler (though not one I'm familiar with) and will prefer clean water, but it's also 18 years old, which, for a modern boiler, is already pretty good going. The efficiency of modern boilers comes at the expense of longevity and you won't get 50 years out of it like you could with the beasts of yore. You could powerflush the system and the boiler could break down anyway. Also, sludge will not cause a draincock valve to leak. The draincock probably leaked because the rubber washer had perished with age and heat (unless you opened it and the sludge prevented it resealing?), so not convinced the sludge has caused any damage (you mention further damage?).
Boiler aside, I can't think of any other components that are especially likely to suffer from a dirty system.
