For the secondary diesel filter... being completely honest...
It was about $450 clams, it adds extra weight to the vehicle, it doesn't add (or subtract) any performance, it's an extra manintenance item, and I can't spot any 'benefit' other than its good looks out of the box- which are now muddy after a few bush trips.
While I can't find any real reason for anyone to shell out to buy one 'right now', if you were to ask me the same question if/when I get a tankful of dodgy fuel out west, I'd probably give you a completely different answer. If I were to make it completely idiot proof, I'd look at installing a water in fuel warning light on the dash somewhere. There are aftermarket parts that can do this, and I think some of the disco models had it too.
A secondary fuel filter is a 'just in case' mod.
There might be some benefits from additional filtering that lead to less pump wear or injector issues... but realistically, it's just an extra layer of defence against something that I hope to never encounter. If you are one of those blokes who expect a tangible return from your modifications, than this might not be for you. Hence why all the marketing for these things are doom and gloom 'your next tank of fuel could KILL your engine' :p

