Hi...
Candidates must qualify to be on the ballot and actively campaigning in 10 or more states
There is some level of campaign contributions received, but I don't recall what it is
Candidates may decline Secret Service potection (as may former presidents, Carter for example)
Homeland Security has the role of determining who gets Secret Service protection.
The Secret Servie is part of the Treasury Department. There is a "plain clothes" and uniform devision. Ther "plain clothes" folks are the ones you typically see on Presidential detail. Uniform division are found guarding the gates of the White House, grounds, and at the Capital Building.
Being part of the Treasury Department they deal with counterfitting and stuff like that. If memory serves, the Treasury took on the presidentoial protection duty because nobody else wanted it. Early on the president had no protection unit. Andrew Jackson fought off his attempted assassin himself with a walking stick on a Washington street. Again if memory serves a regular presidential unit was formed in the late 1880's by the Treasury.
It usually takes several years of other assignments before an agent can qualify for presidential protection duty.
As I understand things (from knowing a former agent) the presidential detail is very intense, emotionally and physically taxing.
See ya!