First off, to warn you, a lot of players really don't like the Nurgle's Rot rules, as they are kind of mean. Some people might like it, but as it slowly eats away at other players, expect some negative reaction.
That being said, it makes more sense to have his disease effect be the same as Nurgle's Rot. Right now, any warband with him has a 1/2 chance or hurting one of their other members, and a 1/6th chance of losing the hired sword each game. WAY too high. No one will want him.
Instead, just use the Nurgle's Rot. Each game, make a toughness test. If you roll equal or under, he's fine. If you roll over, he's at -1 T. If you roll a natural 6, someone else catches the rot. What you give to others, you risk giving yourself.
That being said, he might still cost a small fee. Maybe, 15gc hiring, and 5gc upkeep?
Also, I'd change Speed skills to Academic skills- A guy who needs a crutch and is losing his digits doesn't strike me as someone who can sprint and climb buildings. Instead, give him this ability:
"Lend a Hand: If the Leper learns the skills Haggle, Streetwise, or Wyrdstone Hunter, he may lend that skill to ONE hero during the postgame phase. He may not lend each skill more than once per post game, and only to a hero that does not already know that skill."
Thus, like many beggar figures in RPGs, he's good at scrounging and scraping.