Retrieve archived data
If there's no duplicate data to worry about, adding another 50 GB of data to a 250 GB database is not going to matter much, especially since the business requirements need the archived data in the live database.

Make sure you have good covering indexes. Perhaps you can import the historic data into a separate filegroup and then mark it as read-only. Perhaps you could use a table partition.

There are lots of options to think of. @AMG's answer covers some more business-related questions for your ongoing SLA as well.