most medieval/fantasy assassins from the books i read use crossbows, and long knives/short swords. with the goal being to bury those big nasty bolts for the crossbow into them.
that said, hvy crossbow, and the espanda both work for gear keep it rather light, 12 str and 1 IF isnt going to give you a ton of HP, so arm wont help you. Id stick with the black hood with mask, leather vest, leather gloves (you can go to mail gaunts if you want more armor) and turkish shoes or khergit black leather boots.. goes very well together. In all honestly armor really comes down to personal preference, but id still keep it rather light, think of the armor archers wear.. that sort fo stuff.