Slowly, my company (150 users) is moving to Linux. We want to get off of MS SQL, but the migration task seems like a big deal for a 1-person (me) IT shop. If we chose MySQL, would we lose any functionality or data? Am I going to be able to handle this alone?
A loss of data should not occur if the job is done properly. Functionality may be difficult to move, depending on the amount of stored procedures in your database. MySQL 5 has stored procedures, but MySQL follows the PL/PSM standard for stored procedures, and MSSQL uses T-SQL. While the two standards are similar in a lot of ways, you may find youself having to modify the MSSQL stored procedures to get them working with MySQL.
You can use the MySQL Migration Toolkit to help move your data and schema from MSSQL to MySQL, and I would recommend reading an article I wrote on migrating MSSQL and Microsoft Access databases to MySQL. In addition, I delivered a presentation on migrating MSSQL and Microsoft Access databases to MySQL, and you can view the presentation materials here.