Corian is great if you like the way it looks and feels. Some people think it's plasticky, which of course it is, but that doesn't make it less good. :)

Get an integrated Corian sink if you can--the Corian is attached seamlessly to the sink. If not, at least do undermount (with whatever counter you choose).

Hard as it may be, just wait till you decide to paint the cabinets! Things change over time and counters are pretty much not. Wait, please and just keep redesigning your kitchen in any way you desire...it well be here before you know it....and try to look at model home kitchens to get ideas while you wait.

Susan one of the reasons I wanted to do countertop prior to cabinets is in case they have a problem getting old countertop out and damage the cabinets

I just did Corian (Glacier White) in my kitchen and I'm thrilled with it. I googled Corian Platinum and can't say that I love the color (not a fan of speckly surfaces in any material), but I'm not the one who needs to like it. It is more of a mid-tone, so if you are looking for something to lighten up the space, you might want to look at some of their lighter colors.

I have an undermount stainless steel sink that I'm also quite happy with.