SSD to work with my MOBO

Yes, you can use a SATA 2 SSD with that motherboard but I would recommend a SATA3 SSD. Good quality SATA 3 SSDs can be found that are relatively inexpensive.

The minimum size SSD you should buy if you're going to use it for your Operating System is 60GB.

If you are going to purchase a 30GB SSD then install your O/S on a HDD and use the SSD as a cache drive.
 
Yes, the motherboard or SSD will scale down to meet the needs.

SATA II are going to be hard to find, and no cheaper than SATA III.

30GB is going to be hard to find, start with 60-64GB.

My recommendations, without a budget are: Intel 520 60GB, OCZ Vertex 3 60GB, Samsung 830 64GB, Mushkin Enhanced Chronos 60GB.

Follow Dereck47 advice, and use the SSD cache option of the Z68 chipset.