Welcome Odd Job,
To try and answer your questions:
1. Probably because they are heavy and more difficult to "level". Personally I prefer steel (galvanised) - light, easy to handle, won't rot, but is only slightly easier to level than concrete (unless you use something like uni-piers which are probably easier than timber).
2. Don't know why 50mm, but generally you don't want water from the deck running off and inside your house.
Cheers.
Vernon.
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Bite off more than you can chew and then chew like crazy.