Its looking good - if you put as much work into the birds as you are the loft, you should be very successful.
The perches will work - they'll perch on anything - one of mine insists on perching on the bolt inside the door! ;D You might find that the lower ones get droppings dumped on them : How about running those perches horizontally, then stagger the next row of perches under them??
Will the rain not run off the roof onto the floor of the aviary? Some sort of gutter maybe?
No need to paint inside the loft, just treat the exterior.