I've been offered an escort van and a few grand for my Laguna estate. I really need a van and the few grand if I'm ever going to get started in business. I've got a unit lined up for the end of the month and I want to all set to move in and work.
For a roof rack I welded 'L' brackets to the roof of my Mondeo and then made brackets and bars to bolt on when needed.
If I was making side racks for a van then I'd look to make them detachable...probably fasten to roof brackets up top, but for safety I'd make holes in the van sides low down and clamp the bottom of the racks inside the vehicle. Then I'd have some method of temporarily plugging the holes...rubber or plastic bungs, etc.
Depends if you think you'll be selling the vehicle on and how much it's worth, I suppose.