I had a 400mm telephoto lens for a camera, and a screw-on eyepiece that
converted it to a 40x telescope.
With that, I could just barely make out Saturn and a big ring.
Based on this, I'm guessing you would need at least 100x to get a good
image of the rings.
Magnification can usually be easily changed by simply replacing the eyepiece
of a telescope, so the important thing is the size of the objective or mirror.
Get the largest objective or mirror you can afford.
Then get an eyepiece that will give you the magnification you want.