Yes, your site can be fully public (searchable), public only if you provide people with the link, or accessible only by the WMU community.
There is no direct migration tool. You can create your Google Site and then browse to your homepages site and copy and paste the pages into the Google Site. More information.
That depends on the complexity of your current site. Since Google Sites are template-based, you will need to work within the constraints of the template you choose. Some features that are currently on your website may not transfer to Google Sites, but in most cases, you have more options with Google Sites than with homepages.