Which model for a content suggestion in which only some of the items are currently available
It is pretty common in recommendation systems to have business constraints to filter against before a final recommendation is made (e.g. checking whether each recommendation is in stock).
One way to manage this is to produce a superset of recommendations (e.g. 100 if you need to produce 10 recommendations), apply your filters (which could drop the number of recommendations down considerably), and then return the top-10 recommendations from the filtered set. So, you may not need to produce the full 1000 item recommendation set each time, but you most likely will need to generate a list which is initially large and filter down.
A potentially helpful link:
https://buildingrecommenders.wordpress.com/2015/11/10/the-components-of-a-recommender-system/