"The trees , if I get a professional tree climber can have the pullies at approximately 50 meters up from the ground."
FIFTY Meters up? Those are some serious trees
for a backyard. (Maybe I've lived in the suburbs too long)
I'd go with the 80m ladderline doublet, with a really
strong center connector.
Also I'd use high end marine quality pulleys (from
a boating shop) and stainless steal hardware everywhere
you can outdoors. Oh, and UV resistant Dacron or such for the
ropes & lanyards, also available at the boating shop.)
Not cheap, but if you're paying professional tree climbers
to go up there and set this up, you don't want the hardware to corrode
and/or fall apart after a few seasons and have to pay them to go up again.
Boy, I'd love to see photos of those guys climbing the trees.
Maybe you should build a webpage about how you put up