Well, from what I gathered by actually asking Legion, he acquired the N7 armor when he came across Shepard's final location (because he was looking for Shepard's assistance against the Reapers or something) and either came across the corpse first and decided to leave it or arrived at Shepard's last known location after Liara had given Shepard to Cerberus and left a bit of his old armor behind...not entirely sure which. Either way, Legion apparently got shot at some point and used a bit of Shep's old armor to seal it up as best he/it could.
..that would actualy make scene..if the last known location to Legion was the normandy crash site it wold exsplane why their was no enemy ther cause Legion probably fought them all. and then as you siad he used his armor to repair himself
he was shot when he went to Eden Prime and had a whole that he didn't repair until he had gone to every planet Shepard had along the way until Shepard died there he found the old N7 armor and used some to repair himself