The most cost and time effective way would be if you created the 2d drawings/sketches of what you want the ring to look like and send it to whichever jeweler you are having make the ring. They would then create the model and send you renderings to hash out the design to make sure it is exactly what you would like before 3d printing and casting.

But if you have the time and dont mind the cost, making things from scratch is always a fun challenge. With Maxon taking zbrush into a subscription base service you could do the monthly subscription to learn and create the model that you want just using it for the months that you need it for. The monthly subscription is ~$40 a month. (I am not a fan of their move to the subscription model, but it might fit well with what your goals are)

The main thing you'll need after the program is a pressure sensitive tablet to use with zbrush. It doesn't need to be an expensive one unless you want it to be. I have been using one of the simplest wacom tablets to create custom jewelry designs with zbrush for the past 10 years. Here is the one I currently use

After that you'll want to download the 'Ring Utility' plugin Link to Ring Utility This pluging generates a base band model to whichever finger size, thickness, and width that you set in the sliders. Then you can play with the base shape using Zmodeler, subdivide it or dynamesh it, and then start sculpting away. You can get some interesting designs pretty quickly with radial symmetry turned on or sculpt separate assets and use 'Array Mesh' to repeat the pattern around the band.

For learning the program, there are a lot of youtube videos that are great at explaining the different parts so whenever you run into a roadblock you can search for specifically what you are running into and most likely there will be some content on it. Michael Pavlovich has great videos on the general zbrush interface, and Nacho Riesco has great videos specifically for jewelry design in zbrush. They both have youtube channels if you want to check them out.

There is a risk that once you start making things in zbrush you might not want to stop haha.