It's a bit of work but it's absolutely worth it. Vanilla version is intolerable imo.
Total size of downloads depends on your desired level of immersion. I recently downloaded a face pack which is 11 GB on its own!
What I consider essential: Option File Patch (I did use PTE but then they canceled updates so I switched to PESUniverse, which I know some here have a problem with because it's subscription based), pitch textures, ad-boards and score boards.
Then of course Sider and Evoswitcher. Being able to customize the broadcast camera alone (via Sider) is a reason to get the PC version.
I also have stadium server installed, it's a bit fiddly with installation of new stadia but once you can play at the original stadiums rather than Neue Sonne all over again, you keep forgetting this.
I guess altogether with all patches and add-ons I am hitting the 100 GB mark.