A houzz contributor, not too long ago, had made a wine rack out of 4'x4' wooden pallets, that are used in the US to move a load of goods with a fork-lift truck. Perhaps they have them in your country. He cut them in half so they were only two feet deep and stacked them up to high counter height. The wood would be a nice contrast with your concrete, might add a little warmth to the space. cascio.offsite@gmail.com

Effectiveness is influenced by capacity of unit. If you are trying to mount a 270 cfm fan more than 30" above cooktop, then it may make noise but will do little to clear the air. If it is 1000 cfm, then it can be higher than 30". So the answer is "it depends". Most people don't mount them more than 36" above the cooktop unless it is a really high powered unit.

I mean that I want to put my stove in front of a window on the platform and the chimney or hood ventilation over it .what should be the height of a chimney over the stove .I mean I want to work standing in front of a window.

My go to hood is Vent-a-Hood for all of my high end kitchens. They are quiet, highly efficient, and different technology than any that you suggested above as far as the filtering.