You can't, unless you a Saudi billionaire. These things are completely custom, are hugely expensive (why they have sponsors all over them) and often they will have work lined up for literally years.
You also wouldn't want one. They cannot be driven on the road really as they aren't legal. They will also break a lot. Generally the more tuned a car is the more maintenance it needs.
If you are interested in cars, you are better getting an older vehicle and somewhere to work on it e.g. a garage and working on it as a hobby at the weekends. You will learn a lot more and can actually enjoy it.
You can have one built; you just call Vermont Sports Car lol.
Yes, price is a major factor.
No, you are completely incorrect on street legality; and way far from the truth lol
The basis of a WRC rally car is that it is indeed street legal; and is required to be driven on the public roads with a proper license plate in between the stages of the rally.
While I agree with your comment about learning more by doing the work yourself, you don't need to be a billionaire to acquire one of these. Yes, they are expensive. A typical pro-level WRC spec WRX STI rally car from Vermont SportsCar goes for about $600k. They are actually very reliable though. And thats a bargain compared to just about any modern hypercar.
Rally cars also must be street legal because they are driven on public roads between stages.
You also wouldn't want one. They cannot be driven on the road really as they aren't legal. They will also break a lot. Generally the more tuned a car is the more maintenance it needs.
If you are interested in cars, you are better getting an older vehicle and somewhere to work on it e.g. a garage and working on it as a hobby at the weekends. You will learn a lot more and can actually enjoy it.