One of the downsides to that rule, is that it heavily favors incumbents who have big donor networks in place. And it imposes a big cost on the new players, just to get the donor networks.

It's one contributing reason why it often takes a self-funded millionaire/billionaire to unseat them - or get real traction in a campaign.

Hence why public funding for candidates is so important.

What counts as a campaign expense? If he buys a new suit for an event he just committed a felony?

No. But if he pays for an entire tech team in Manhattan to run his campaign, he should (under campaign finance regulations). I did a phone interview as a potential candidate with someone on that team because I was curious about their capabilities. Such a team should not exist for a self-funded candidate.