What should I build a bench out of?
Ok, I did the metric to inches conversions so I understand the size of your project. Sure wish the USA would go metric like the rest of the civilized world. LOL.  

Your bench is going to be apx 7 foot by 2 foot with 17 inch overhangs. I think you will find that a solid stable material like 20 mm (3/4inch) plywood, medium density fiberboard (MDF) or solid boards will work well if the load is not too heavy. Avoid materials like cheap chip or particle board  because they have a tendency to warp easily if not secured to a sub-straight. Because the surface is not real smooth, they don't look great when painted. 

If you want to be sure the unsupported overhangs don't curl or distort, you could frame under the exposed sections with an extra parameter rim of say, 2 to 3 inch by  3/4" solid stock mounted flat under the overhang. This would effectively make the overhang 1 1/2 inch (38mm) thick. 

Use a good grade stain blocking primer and finish your project with a couple of coats of quality enamel paint.