How strong will the strap be? How much weight will it hold?
This usually depends on the hardware that is selected. Hardware is rated with a breaking strength and a working load which is usually 1/3 of the breaking strength. You should select strap hardware based on working load, not breaking strength. Frequently, if the load applied approaches the breaking strength, the hardware will be damaged due to mechanical deformation, even if it does not break. If sufficiently strong hardware is selected, then the stitching on the webbing becomes the next weakest link.