How can I support a heavy sink in a homemade bench table?
I think the general bench idea is fine, but depending on the actual weight and positioning of the sink, it might need a middle leg to support the weight.  My plan would be to build wooden framing that would support the weight of the sink on its own so the choice of top material wouldn't matter as much.

In other words, build a top with a framed out rectangle of boards that would fit the sink and support the full weight.  For the choice of top material, think about what is going to be durable in the possible wet conditions of a sink.  Plywood would work fine, but will need to be sealed well against water.  Wooden slats might work well also, but again it will be more of an aesthetic choice if the frame is made (or reinforced) to hold the sink on its own.

Just general guidelines - for a 6' long bench and a 60lb sink, I don't think you would need a middle leg if using 2x4's for the "frame" of the top and the sink on one side.  If you needed longer and the sink is moved more to the middle, you might want a middle leg.