Corpses disappear after 72 in-game hours. Unless you visit the cell again before that, or the corpse has Corpse Persist toggled on. (but very few corpses have that)
Your safest by just leaving the items on the ground. They will not disappear.
If you're on PC, and you know the ID of the item you want, just use the console.
Player->additem "ID of item" 1The TES wiki has all the IDs for the objects in the game. Or check it in the CS.