Begin the China mission and complete the first objective for $30,000. Then when you have completed the objective, quit and reload the mission to get another $30,000 (to get the money you may have to walk back down the mountain). This can be repeated as many times as you want.
Anytime during Campaign mode go to 'Start New Campaign' at the 'Campaign' menu and choose the character you want to play as. Then return to the 'Campaign' menu and select 'Continue Campaign' and you will start with the character you selected.