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.
Bite off more than you can chew and then chew like crazy.