SSIS ignoramus needing help!

the situation: a temp table is populated from an excel file, which has been known to change formats at random times, that is owned by a different group. a lookup need to be performed on the temp table, tableA, to populate tableB with valid data. if the lookup results in 0 rows being returned, an email should be sent and the existing data in tableB should remain untouched. If the lookup results in a number of valid rows > 0, tableB should have all rows deleted and the new records from the lookup on tableA inserted.

question: what would be the best way to check if there are any valid rows and perform the appropriate action(s), depending on my results?

Thanks!