Our smallest portable system weighs around 25lbs and can mount to just about any vehicle using strong magnets or standard roof rack. It pulls less than 10 amps so it can be powered through a standard cigarette lighter outlet or wired directly to the vehicle's battery.
Our largest system weighs many hundreds of pounds and mounts on one specific vehicle in as many reinforced locations as possible. We utilize factory bolt holes on the roof, make custom brackets to support cantilevered weight off the back, and make modifications to the roof up front to support even more weight. The system typically pulls 40-50 amps however, the vehicle comes with a high output alternator and dual battery system from the factory so we don't need to upgrade them.
Based on the components of the current Street View system, a roof rack is sufficient to support it's weight. I'm guessing it pulls around 20 amps so it likely has a direct connection to the vehicles battery, but wouldn't require an alternator upgrade.