If I were you starting out with rulers, I would buy the Westalee starter set for your low shank machine, and then perhaps try one of the 1/4" multipurpose rulers like this one, which is not expensive and you can do multiple designs:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Just keep in mind you will only be able to use the thicker ruler in front or on the sides of your ruler foot, which usually is doable. This way you will get a feel for both without spending a great deal of money, and you will be able to use the 1/4" ruler should you get a mid arm machine.