There is no limited number of products to be cached.
Yes, it is.
Yes, Magento 2 Full Page Cache supports multi-store as well as multi-language
The APC cache is the fastest cache. Memcache is needed in a multi-server environment. The file system is for all customers, which have no APC or memcache installed on their server. You can only see APC or Memcache as option, when these are correct installed.
Yes, of course.
It is fully compatible with Magento One Step Checkout extension