One thing i don't see mentioned is the effect on the floor you're going to be on. 3000 LBS/8 wheels, that's 375 lbs. Not too bad, right? Now, the contact point of the wheel on the floor is going to be somewhere around 1 or 2 square inches. That's 180-370 PSI. The equivalant of focusing the average weight of an adult male (or more) on a postage stamp. Larger diameter wheels and softer wheel material increases the contact point to better distribute the weight. Most wheels i've seen for this type of weight are for industrial applications so they're hard plastic rather than rubber, which reduces the contact point. Your preference is to make the tank lower, but that implies smaller wheels too, smaller contact point.
You should probably have a steel frame that the casters are on rather than trying to put the casters directly onto the cabinet since cabinets aren't designed to be supported by a few contact points but rather distribute the weight across the floor evenly. I wouldn't try this on a normal framed floor in a house, just concrete. Don't roll too fast.