The standards are forward/backward compatible and for HDDs. SATA II is adequate, as HDDs do not saturate the connection to make SATA III useful. Therefore, I would just get a SATA II (or III if same price) drive of the capacity you want. It uses a standard 9.5mm height drive like this one, or any similar: http://www.seagate.com/files/staticfiles/support/docs/samsung-ds/100698122c.pdf
You might also consider an SSD in the main bay and moving your HDD to the spare slot, which would give you an incredible performance increase.