I don't think you'll get the actuations with leading zeroes. Adding a letter for the model can be achieved via a code replacement.
Simple create a code replacement file with the model name, a tab character, and the letter you want this model to get. Then set your renaming string as follows:
\{model}\{act}and you should be done
Your Code Replacement file could look something like this (make sure to use a tab character before the replacement letter!):
// Photo Mechanic Code Replacement File
// Camera Model
NIKON D3 y
NIKON D300 e