A SSD will set you back a bit if you want large storage. I wouldn't recommend that kind of investment for this older system.
A better solution would be a hybrid (SSHD) HD. These SSHD drives are a standard spinning disk drive with a SSD acting as a cache. Seagate makes a 1.0TB drive that will fit your system and also run at the slower SATA II (3.0 Gb/s) I/O speed you'll need here.
Your system can support any 2.5" 9.5mm high drive that is SATA II or can be set for SATA II. I do recommend updating the EFI firmware. Follow this Apple TN on how to check it EFI & SMC firmware updates for Intel-based Macs.