As for your requirements and other suggestions here... C24's are out of your price range likely unless you go finding a use set.
Independent testing, such as the Tour Magazine have shown the difference between a good aluminum alloy setup and a cheap aero carbon is often nothing. The Zonda's come within 2-3 watts of many ~45-50 mm deep carbon wheels that cost 3 times as much. The older Kinlin rim beats many of them by a couple watts. What's not to like about that value proposition?
Disagree, 30 seconds with google shopping shows C24s for £639.99 from the UK - Mine came without VAT as I'm in Switzerland.
Agree regarding carbon / aluminium wheels - I have Mavic CCUs, Edge Carbons and so on in the shed, but choose 99% of the time to ride the C24s as they are nice and stiff, comfortable, roll nicely, bombproof but best of all they brake really well in the wet down hill, which is much more important than weight in my experience - Uphill a 10km hill (lots here) you lose a few seconds on weight and gain maybe a minute because you can brake confidently into every corner. On dry sunny days the CCUs are nice though.
C24 clinchers are not stiff. They aren't noodles, but aren't a stiff wheel. You should know that with CCUs!
My assessment is based on personal experience, the stiffness tests Tour and others have done, and the experience of other owners.
I also think "bombroof" is generous, based on the same factors except the Tour tests. They aren't bad durability wise, but below all the wheels I've had from Campagnolo for instance.
Where they excel is the lightness of rim and comfort - agree with you absolutely on that.
Stiff, Light, Aero - Pick Three!!