Crossbars will pretty much handle any weight, it is the attachment (towers and feet) that are load rated. Yeah you need them some what substantial, if you haul another with you and both have 80 lb kayaks, you are at 160 without any gear.
IMO only two brands to look at: Thule and Yakima. It is pretty easy to find used crossbar and towers, just need the specific fit kit for your vehicle.