How much light does it get? Slate could complement walnut nicely.

@NG sintered stone is like a porcelain and so the edge and the sink cut out will be plain and polished. In the same way any drainer grooves will be plain plus most sintered stone is so thin that the drainer grooves probably aren’t very deep or very effective. Speak to your kitchen designer to find the best option: Over mounted sink with drainer Flush fit sink with drainer Under mounted sink with drainer. Multi level sink with accessories that act as a drainer. Sink and drainer manufactured in the worktop material

I think it's personal preference. I'm going to have an upstand and a splashback. I'll try and attach a pic so you can see what it would look like (this is a mirrored splashback rather than glass though)

Best to shop around, the mark up some showrooms put on worktops is outrageous, especially for solid surfaces like granite and quartz etc. I prefer quartz for worktops as there's such a wide range of colours to choose from but always choose a quality brand name as recently some of the cheap imports have been found to be not food safe. For durability and stain resistance have a look at http://www.fitzgeraldsinteriors.co.uk/which-worktop.html these test were done before products like dekton and solid laminates were popular though.