If you buy a proper and suitable bike there is no reason to have to change the stock components until they wear out after several years and thousands of miles so eliminate that premise from your thinking.
As others have said, building a bike up from parts is always more, usually much more, expensive than buying a complete bike.
Finally, a complete novice to bike mechanics is very ill advised to take on a complete build-up project. You don't know enough to spec the suitable parts and don't know how to install and adjust them. Buy a bike that suits you and learn to work on it over time.