First, decide what size you need. Do you need a single bowl or double bowl? Then how big you want the sink? After that it's the depth of the sink? Before you decide on your budget.

When you have decided on the above criteria, next is to check out the brands, feel them and make a few sink choices based on your criteria and budget. It will be a matter of choosing among the few sink choices and which you like most. Lol, isn't it simple?

As for SS grade and thickness, be sure to ask seller about it if you cannot find the information of the sink specs online. Google is your friend to research.

The thickness measurement I believe has been discussed in lowyat forum here. Do a search.

Yes you can buy sink in advance, just tell the contractor the sink dimensions and faucet size or just show him your sink and faucet to measure properly.

Just managed to find the single bowl granite sink which in my opinion is vvv slightly more superior than HCE so there it is sitting in my car boot now 

haha ... The one I bought was claimed to be made by the same factory and they do look identical. I inspected the quality which is good and to my liking. When I mentioned it's vvv slightly more superior, it's because the filter strainer is better and the price point is slightly cheaper (cheaper also partly because i bargained hard haha    ). And same 10 years warranty so why not? haha

it's a granite sink which i don't think you are looking for this type

top mount ss sink is the most abundance