As a reference point, I have one 300 watt panel and it is more than enough to recharge my previous nights usage within a few hours of sunlight. To understand my usage, I have 400 amp-hours of lithium battery, a 7 cubic foot fridge, an induction stove, two fan coils, and several large lighting circuits (two at roughly 20 amps). So the number of panels you are planning to install begs the question: are you electrifying the drivetrain or using this to charge some electric racing motorcycles? Because you have way more generation than you'll use without a load much greater than the typical Sprinter camper.
As for building your own roof rack, as others have said, it really does matter, but I agree that building with aluminum extrusions would be fairly easy and require no welding skills, just wrenching. If doing this, really think about your design and how to connect all of the pieces in a way that will create the right balance of strength, usability and weight. The key to weight, cost and usability is on the connections more than the overall design. Welding one as others have said is going to be a lot easier with square tubing than round, or bar. You can also bolt together square tubing or bar fairly easily and perhaps at a lower cost and greater functionality than bolting extrusions together.