The Dutch trains journey planner shows which platform trains depart from. Note that platforms are often delineated in to (say) 14A and 14B so you need to be in the right part of the platform to prevent a last minute dash to the right part of the platform when the train pulls in.

The cost of the fare is €5.30 (which includes the €1 fee for a paper ticket). If you use a credit / debit card your provider may charge you foreign exchange fees. I have never been charged extra for using the ticket office over the machines. It's a good way to use up any € coins you have before departing!

You can now buy an e-ticket which avoids the €1 fee but it looks like you can only do that when you are actually in the Netherlands - it worked for me when I was in Amsterdam but not when I tried to do it from the UK

I stayed in the Ink last weekend and thought it a very good hotel and 5 minutes walk to Centraal. Turn right out of the hotel and just keep walking!