Hiring a professor to review doctoral thesis
Do you know the old saying that you can have something done 1) cheap, 2) fast, and 3) good - but you can only pick two of three? That applies here.
There are not many professors available for hire for something like this. If you can even find one, I would suspect that they are not very good. Good researchers are generally busy people, who would not want to commit to proof-reading some strangers' thesis. Unless, of course, you are willing to pay a large amount of money. I am not at professor level myself, but I am qualified to read a thesis and provide comments. If you wrote and asked me, I would probably say no, because of the obvious ethical problems. But if I entertain the thought for a moment, I would ask my standard rate for external consultations which, depending on who is asking, is 2-300 €/hour. If you are willing to pay something like this, then it is a matter of sending someone you know to be good an email and asking. Expect many people to say no before someone accepts.