15v and 18v are not compatible
The 12v in the recon sale is the best course and it comes with 2 batteries and a systainer
I have 2 older C12v drills and they're awesome.
All the Best
I have a C 15 and the only batteries that work are the BPC 15 batteries. Neither the older BPC 18 or the newer BP 18 batteries fit in the drill. The drill still works fine despite both the batteries and the drill being over 13 years old.