For A rank, you have to collect all total of 100 leeches to get infinite ammunition for all your weapons in the next game.
For B rank, you have to collect around from 90-99 leeches to get a magnum revolver which you should find on the train where the passengers' cabins are.
For C rank, which you collect from 80-89, you get a hunting gun with infinite ammo, find it at the usual spot and it will be loaded with unlimited ammo.
For D rank, which you collect from 50-79 your handgun will be equip with unlimited ammo. Check on it when you restart the whole game after you beat the Mini Leech Hunter.
For E rank, collect around 29 or 31 and you should be able to find a sub-machine gun.

That's all I know even though I haven't been able to beat the whole thing yet!!!