maintaining full text index on large table
Full text in SQL Server should have no trouble keeping up with your workload.  I currently support a full text index in SQL Sever that is  approximately 500,000  rows with 125,000  rows being inserted and deleted everyday.  Query load peaks at around 200 full text searches/sec. Response time it's fairly consistent in the .5 to 1.5 sec range.  

Recommendations for full text


Use auto change tracking.  It just works and you don't have to schedule anything.
Create sql agent alerts for full text population errors.  Populations can fail (they rarely do).  When they do fail you have to resume them manually.  Having a sql agent job kick off an "ALTER FULLTEXT INDEX ON myTable RESUME POPULATION"  makes it a non-event.
Consider trace flag 7646  if your query load and the volume of updates are both really high.  This flag is applicable to 2008/2008 R2 (not 2012).  It reduces some blocking on internal data structures and was documented in a SharePoint best practices document. (You probably won't need this)
Otherwise, you probably won't need to do any additional tuning.  It typically just works.


Sql 2012 introduced substantial improvements in scalability and performance  so upgrade if you can.

As for how to manage the inserts and updates, just update the rows add you would normally.  I wouldn't recommend truncate/reload, but otherwise don't worry about it.