I agree with Prism, you need more than the one ruler you listed. It is important to square up your blocks as you make them. Get as many different sizes of square rulers you can find, all made by the same company.
Don't forget to buy a lot of blade refills for your rotary cutter.
Seam ripper. Unfortunately, we all need this tool.
You said you have scissors. Large or small? You really need both sizes.
Quilting safety pins for pinning the quilt sandwich.
I also think you should get a variety of fusibles.
Have fun on your shopping trip and good luck!!