Concrete bench tops and decorative concrete pieces are a good idea if you like the solid look. I certainly would not recommend asking a general concreter about a concrete bench for your kitchen. Normally the bad rap that concrete bench tops get are when they are manufactured like a driveway or footpath. Kitchen bench tops require different characteristics. They should be constructed dense (to help prevent staining), flexible (to prevent cracking), lightweight (so can be manufactured off-site). With the materials that are available now, concrete can have all these characteristics. Sealing is another topic for the professionals. There are three different methods of sealing concrete. Penetrating, Topical, or Reactive. Sometimes all three are used to seal a concrete kitchen bench. This is because the way a kitchen bench is used is different to a floor. You are not only trying to prevent staining, you are trying to prevent etching that could occur with acidic foods. There are companies around that specialise in concrete bench tops for kitchens. Try them you may be surprised at the options available for "concrete" bench tops.

I like the bottom left one in the first pic of the set of three you posted. What colours do you plan in rest of the house? The second sample in your first post has some nice colours that you could pick up as accents.

We have black pearl granite. Been in around 10 years and looks as good as the day it was installed. Easy to clean and doesn't seem to mark or stain. I would however not use black again as I think white would of looked much better. Granite being a natural product can't be matched up as easily down the track if you decide to add more benches and the like. Each batch can potentially be different.

White-matte cabinets (flat panel, not shaker) and white bench (or maybe epoxy sealed marine ply??...siriuskey can tell you about that), mirror splash. Love the ceilings. I personally would go for a modern, stream-line, no handles, integrated appliances look. This space has real potential to be very cool, even with the slate. It doesn't need any more more texture or darkness. This is a nice kitchen, you should be able to sell it on ebay.