In regards to Question #2 - "how much would the rental rate go up if the Landlord construct an enclosure", there can be a couple of potential scenarios.
Scenario #1: You could inform the Tenant to do all of its own build-out itself. In this scenario, you would require the Tenant to submit the plan to you first for an approval before they move forward with their own build-out. However, although this may be an "ideal" scenario for a landlord, I do not believe this particular prospect would be a fan of such proposal based on our interactions during the first couple of meetings.
Scenario #2: I believe the enclosure that they are asking is not going to be that expensive. I am guessing maybe $2K to $3K at a retail price.