I would make a copy forum (under the licence as a private test site) - do the deletion of other forums threads users etc...there and then do the import.
In the copy you may need to create a new forum node if the one with that thread in has the same ID as any forum nodes in the target installation. Not sure what happens if there are , but best to be on the safe side.
You wouldn't be able to retain IDs when importing into a populated forum so the node ID is irrelevant.
Deleting users would be difficult as you would need to keep any users who have posted, liked, been quoted, reported, or performed any other action on the node content.
You would also end up with invalid attribution links for quotes from any other forums, etc.