SQL Server replication is the obvious answer for "fast, light, and easy" on both servers, but that implies getting administrative access to the MSSQL server.
If you can't get admin access to the MS-SQL Server, then I'd use BCP to create a text file from the source tables in MS-SQL. Create a staging area in MySQL with two generations of the MS-SQL data. Compare the two copies in the staging area and use the results of that comparison to maintain your MySQL working tables. Uglier than the south side of me heading north, but it would get the job done with the least possible effort and reasonable storage.
-PatP