When you say CAD and rendering, if you are doing mostly CAD focused and some rendering using mostly automatic setups and default settings with little adjustments you want to use a CPU with the highest clock speed regardless of thread on the CPU.

If you are more rendering with some CAD and your rendering requires multiple iterations with you customized each rendered scene then go with the most threads that you can get, or the Ryzen AMD line for the most threads. Of course, some rendering programs will also utilize GPU cores as well so it could also depend on your rendering software of choice and I would ask them if they use CPU, GPU, or both.

The reason, at least from my knowledge, is that most all CAD functions cannot be performed in parallel but in series, meaning that A has to be completed on a single thread before B can be started.

But with rendering the job can be divided up among all threads, thus the more threads the better even if you are on a slower CPU.

For heavier CAD work, go with the fastest CPU regardless of AMD or Intel based.

For more customized scene rendering (as opposed to drop in your models, hit render then go get a cup of coffee while it crunches) than CAD go Ryzen 1700 series for the 8 cores and 16 threads.

Now that is general and will work for most, but make sure to contact your CAD and rendering software makers to verify if their CAD package works on multi-thread (again most will not). Also make sure that you are talking to someone in the know. With computers, like cars, people often think that bigger or more powerful everything is best when with softwares that require specific things to work best, the most powerful CPU may not give you hardly any advantage or even lose to a mid-range one that runs at a higher clock speed.

Best advice, no matter which way you go - plenty of ram since it is cheap. Think 32GB. The worst thing with any CAD tool that I had problems with was working on models or assemblies that would constantly hit the harddrive as a swap file since I would often peg the ram.