Yeah, you could use the free flight standalone gps for the transponder but thats really money going onto only one item with no other benefit, I would rather pay a bit more and end up with at least one 430w then I get a new radio and nav or possibly you could use a 400w so no radio or nav with that one.
On the plus side the avionics guys are going to be able to do an instal after all this with their eyes closed.