Looks good,
French drain would do fine. If the old global warming thing is
correct, you could be looking for water, so if an option would be worth
having some tank arrangement. Don't for get to allow for some drainage
from an overflow.
I don't think you will go far wrong with 100kg m sq, but after you have
worked it in you will have a feel of how good it is. So long as it
isn't too wet, working all that sand in is a good job for some meaty
rotovator from the hire shop - the kind with wheels
Good luck
Clifford
Bawtry, Doncaster, South Yorkshire