If I buy a 3d model at cgtrader.com, what software should I buy on Steam to render it?

What are you trying to do? You could try Blender (free) cycles renderer and see if that meets your needs before doing any paid options.

It depends on what type of images you want to render, do they need to be 100% photo-real or is it enough they look cinematic, maybe they need to look more cartoonish, etc.?

There are many types of renderers and they all provide different features depending your needs.
For example a production renderer has features for multi-pass rendering, a renderer that is more geared to making a static image may not have this features, etc. There also is a difference in off-line versus real-time rendering, real-time renderers trade image quality for speed versus off-line renderers trade speed in favor for image quality.

For example Marmoset Toolbag (https://marmoset.co/toolbag/) is a renderer geared towards fast rendering with preconfigured PBR materials. Its very easy and fast to setup and renders nice images in real-time and its inexpensive. I use it allot for presentation images but when it has to be more photo-real then I use Octane render (https://home.otoy.com/render/octane-render/), that one is also easy to setup and it renders relatively fast if you have a powerful GPU, they also provide plug-ins for integration with most popular animation software and its not to expensive.

Vray (https://www.chaosgroup.com) is probably the most popular renderer and lots of models here are setup for it, that can be advantage. However, Vray is a bit more expensive and maybe you do not need its vast amount of features and also consider its not as easy to setup as Octane.

Personally I moved to Octane because Vray felt cluttered with lots of stuff I do not need and it just wasn't that fast to setup (that was at the time I made the move some 3 years ago, maybe things have improved in meanwhile?).

If you need to render in real-time then Unreal engine (https://www.unrealengine.com/en-US/what-is-unreal-engine-4) can also be your thing and its free. The things you can do with it are limitless, its so packet full of feature its gonna take some time to take it in. Octane render will soon release a plugin for Unreal engine so that's also good news.

If you have no experience with rendering and with 3D in general, i would suggest to consider comissioning skilled person for the job. Even though 3D became much more accesible over past years, it still requires significant amount of skill and experience to get decent results. It's not as straightforward as buying/downloading few models, a software for rendering, plugging everything together and pressing single button to get a beautiful artwork.

I think that the essential thing for a render to look photorealistic has to do more with the lighting of the scene and the lights, more than the textures, but it really helps a detailed texture.

直接渲染

渲染器啊