Enterprises dont want people to be exclusively dedicated to build pcs. They contract technitians that review their products when they failure, fix problems and as last job they also build pcs.
Building PCS its not a profesion, its just a part of a tech-worker inside a computer store, and its only a minimal task.
I kinda recommend you to first study and get the career and then try applying as tech support in a computer store if building pcs its a thing you like.
As for the income much depends on your country and how much hours do you work for them. But its not quite a very high salary if we are valuating all the time it takes you to get that carreer and develop your profession.