The actual loading capacity is going to be dependent on what size "workbench" you end up fabricating with that kit. The bigger you go, the longer your spans are going to be, and the less capacity you'll have.
Building a custom frame is easy. Good rule of thumb for aquarium stands is to keep all your spans no longer than 2'. Also, you want to use compressive loading. I've seen a scary number of stands built that rely on the sheer strength of the fasteners (generally screws) used. Bascially, you want the weight of the tank distributed DOWN to the floor, and never sideways.
Where things can get complicated is if you want your stand to look nice. Then you have to start looking into skins (panels, plywood), trim, stains, paint, etc. It is what you make of it, though.