The ABEC specs have nothing to do with durability or sealing. You can find sealed ABEC1 bearings, and ABEC7 bearings that are not sealed. The ABEC ratings only specify the tolerances of the bearings. It all depends on how accurate you want your machine, but since even ABEC1 bearings have runouts less than .01mm or .0004 in, just about any of them would work for most machines. Keeping water out may not be a problem, but I can assure you that keeping dust and dirt out is important, so I would look for sealed bearings. For your reference, this page lists the tolerances for the different ABEC ratings.