Have a look at
http://www.aussieveedubbers.com/forum/viewthread.php?tid=4445
and scroll down.
I used an ex-telstra ladder rack. Three heavy cross bars, the sides are aluminium angles and 15mm ply in between. I can happily walk on it. I aligned the three cross members with the main supports up to the roof from the floor in the body work, just behind the front doors, behind the sliding door and just in front of the rear vents.
I believe the roof carrying capacity is similar to the L300 van the rack came off at 100kg. With ~60kg in the roof rack that doesn't leave much. This is for a dynamic condition and for driving stability, and it will easliy hold more when you are parked. It also keeps the sun off the roof.
As for getting up there, I never considered a ladder as I have no trouble stepping up from the front guard (open the front door).
Andy
:thumb