Just adding my two cents here, post work to me is a complement to the render and also a way to fix small stuff(like poke througs, textures, etc)
If you take your time to set up your lights, enviroment, model, etc. 95% of the times you'll get a very good render, post work can polish it even further. Maybe the lighting, colour, contrast on the render didn't came out the way you intend you can work that on post production.
Just to give you an example if you look at Mr. Vargas render of widowmaker(looks amazing by the way) a few posts up, you can tell what he actually did in post to give his render a more polished look, to give it a little more "oomph" but I'll bet you money if he showed a non edited version it would still probably look great. And yes photoshop is expensive and it's business model isn't consumer friendly at all, but GIMP exists and it has a new update which actually is pretty good not gonna lie. So download it and take it for spin, don't be afraid to experiment with stuff and if you're lost and need help on how to use it, there's alway youtube and help section on the forum.
Sorry for the lenghty post, if it brakes any rules fell free to delete it.