Iron Man II was James Rhodes, III was Teen Tony, IV was Victor von Doom. Other temporary wearers of Iron Man or Stark armor include Weasel Willis, Eddie March, Happy Hogan, Carl Walker, Bethany Cabe, Riri Williams, etc.
Tony Stark's history is fraught with health problems. His armor was originally a pacemaker and electromagnet to keep shrapnel from penetrating deeper into his chest and heart muscles. He received a transplant of synthetic heart tissue in Iron Man (1st series) #19, which eventually led to him not needing the chestplate at all anymore.
His health problems returned when Kathy Dare shot him through the spine in #242, leaving him paralyzed from the waist down. A bio-chip implant in #248 restored his mobility, but it was later revealed to be compromised by Kearson DeWitt and the Marrs Twins, allowing them to remotely override Stark's nervous system. Armor Wars II from #258-266 covered this affliction, leading to the revelation that he was suffering from a degenerative and fatal nerve disorder thanks to his enemies. Stark faked his death in #284 to go into cryo-suspension until his team could find a cure. Stark received an entirely new artificial nervous system, mentally programmed by him, in #288. However, this Stark was mentally corrupted by Immortus and died in Avengers (1st series) #395.
Tony Stark was revived from the ashes out of his teen Tony and Heroes Reborn counterparts in Heroes Reborn: The Return #4 and Iron Man (3rd series) #1, mentally clear and absent and health concerns. However, he suffered a heart attack fighting his Sentient Armor, who experienced sympathy and fed an artificial Ultron-coded heart into his chest in Iron Man (3rd series) #30-31. Stark once again relied upon outside power sources to charge his heart.
Stark completely rebooted his physiology with Extremis in Iron Man (4th series) #5, giving him perfect health and enhanced interactivity with his armor. When the Skrulls implanted a virus in StarkTech in Secret Invasion #2, however, Stark's Extremis physiology crashed, leaving him damaged and unable to access his full capabilities. Over Invincible Iron Man (1st series) #8-24, Stark completely wiped his own mind, then rebooted by installing a RepulsorTech arc reactor in his chest. He was self-sufficient and enhanced again, but damage to the reactor would shut down his entire physiology.