Antonio, if you are looking for the constitutional comparison, Citizens United is not a good choice. I think that CU followed fairly logically from Buckley v Valeo (1976), and that should be the focal point of concern for those concerned about campaign spending. In Buckley SCOTUS held that individual expenditures could not be limited since money was required to exercise free speech. Buckley also identified the importance of political association as well as political expression. If political association is as important then a group of smaller donors should be able to band together to compete against a wealthy individual. CU just identifies that associations acting through a corporation are such an entity. The concern should be whether the Buckley holding that essentially money equals free speech is appropriate.
Yes. And I would be amazed if anyone would disagree that billionaires spending that kind of money on themselves in a campaign, while nobody else effectively can, because of limits on raising or spending money on campaigns received from others, is a complete non starter. As it is, my impression is that about half the Senators have net worths in excess of 10 million dollars. That's a real public policy problem to the extent the campaign finance laws foster that.
If I had my way, we would work more towards a system where if a candidate spends over a certain amount, the government cuts checks to the opponents that is for more than the amount of the spending excess. That still leaves what to do about all these "independent" expenditures. For that, there may be no real solution.