What is the best shipping method to test out Amazon UK from the US?

A good starting point for determining valuation is 3x your annual profits. That said, you may find a 1 - 2X variance up or down depending on the type of product(s), the category or categories you sell in, your total number of SKUs, etc.

Yes. https://docs.woothemes.com/document/configuring-woocommerce-settings/

You have to see how much gross profit and net profit is there in the product sale. Let's assume the net profit is X then you could ask around 15% to 20% of the net profit. It also depends on how much efforts you are doing and who is paying for the paid ads. It also depends on how much efforts client is doing to build his brand on social media and other channels because if he is doing efforts, there will be more sale of goods.

Heard good things about GoodsCloud (www.goodscloud.com)

Can you give more information on what your product is? Then we can figure out what you need in it. Sounds like electronics...but what?