You seem to have answered your own question. If a SATA III card is available but not in the budget, you might as well get SATA III SSD now and add the card later.
SATA II data transfers over the connecting cable occur at a faster rate than the internal speed of a HDD. SATA II cable transfers are slower than the internal speed of a SSD. This is why SATA III created.
HDDs internally have a transfer rate of about 200 MB/s, where as SSDs have an internal transfer rate of over 400 MB/s. When a SSD uses a SATA II interface, the actual transfer rate is limited to 300 MB/s. When using a SATA III interface, a SSD is not limited by the interface itself.
Therefore, there is about a 50% performance improvement when a SDD replaces a HDD using a SATA II connection. But, to truly see an improvement, you will need to SATA III connection.
Drive manufactures now offer drives that include both SSD and HDD components. Externally, the drives appear as single devices, but internally cache frequently used data in the SSD. This can give SSD performance at HDD prices. These drives are often referred to as Solid State Hybrid Drives (SSHD).
Apple offer their own version of SSHD technology. These drives are referred to as Fusion Drives. The difference is that OS X is aware the the Fusion Drive is actually a HDD and SSD paired together. When a Fusion Drive is installed, you have to use Core Storage when partitioning the drives.