Welcome!
There are a whole load of threads on the forum about building a render farm, so have a browse through and I'm sure you'll find lots of similar questions and answers!
From my personal experience - it was super easy to setup a render farm for Corona. To answer some of the questions:-
What are the best builds ? Since corona is cpu based, we could buy cheap gpu`s only to have video ? Should they have 32gb or 64gb of ram ? Is core i9 9900 enough or should we buy xeon`s ? Or maybe ryzen ? We need to buy a special license for this or we can only install corona trial for DR ?
No fixed answer on best build... definitely CPU based, all of my farm machines have a very basic GPU.
32 vs 64 RAM etc is dependant on your scenes, complexities etc - my workstation is 64GB, render nodes are 32GB - but I have once or twice found they had issues on
very large scenes. Ideally go for something you could expand later if needed?
i9 vs Xeon vs Ryzen will come down to cost of build, availability, etc - my render farm has Ryzen chips in, but my workstation is Xeon... at the end of the day it's just how much CPU power you can get for your budget.
Licensing depends on how you are licensing Corona - pay monthly will have either 3, 5 or 10 render nodes per workstation licence. If you have a Box licence then you're limited to 3 nodes per workstation (I'm sure someone on staff will correct me if I'm wrong here).