Well, if I understand correctly you want it to be like a rapier, only slashing instead of piercing. That means it's not mechanically the same as a rapier -- almost, but not quite. Is this distinction important to you? If not, take a rapier, call it a cutlass for fluff purposes but a rapier for crunch purposes.
On the other hand, if it is important to you that it be a slashing weapon -- is there any SRD weapon whose stats, taken in toto, would agree with what you want your cutlass to be? If so, take that weapon. For fluff purposes, you call it a cutlass, but for crunch, call it whatever SRD weapon you're using.