I have lived in two places with breakfast bars in American style kitchens. I found that I tended to spend a great deal of time sitting there so a bar stool with a comfortable back is important. I made mine from pine, copied from some I saw in a bar. I was also given an old captains swivel seat from a yacht so I built a stand for it, reupholstered it and unfortunately left it for the new tenant who liked it. That was one of my favourite seats, to sit in comfort at a breakfast bar rather than balancing without a back to lean on.

If you want the seats to swing out from the island be sure to check that the island is substantial enough to take the load, also that the island is well anchored to the floor. If you consider the arm is for example 2 feet long, then sitting on the extende seat will be the same as applying your full weight to a crowbar, two people sitting on one side would destroy a normally constructed island. Similarly, if fitted to a wall make sure that they are anchored well and that the wall structure is suitable. A nice idea to save space though

That height should work fine. If you are UK based, can I also suggest you look to atlanticshopping.co.uk for your bar stools? They specialise in them and have a huge range at very keen prices. There is a showroom (Birmingham direction, I think) if you want to see the product. We use them ourselves for the showroom, and many of our customers have used them and been very pleased with quality & value. If you are buying online, you should still have consumer rights which allow you to return product (just the postage cost lost at most) as long as the product is not "made to order" - in which case make sure you are sure before you buy!

Google says potterbarn.com do, but not here in the UK. Houzz shop is just links to other stores.