Does the pan have the curb cast as part of it? The whole pan up to the top of the curb needs to be waterproof. Typically, they aren't installed with a liner.
If the pan does not have the curb as part of it, I don't know how you'd end up with a waterproof structure.
In a conventional shower, the liner would go over the curb and be fastened outside with the liner also going up the inside walls at least a few inches above the curb.
The junction betwen a manufactured pan and an add-on curb would seem to be problematic in trying to keep things waterproof and not get water accumulating underneath.
A liner should NEVER be installed flat on the floor, regardless of the type of pan construction. The top waterproof layer (and in a tiled pan, the tile is not considered waterproof) needs to slope to the drain. In your case, the cultured marble is waterproof, then to put another waterproof layer underneath it is asking for troubles. The only way to make the bottom liner waterproof is to use a clamping drain, and then there'd be no way to properly seal the marble pan to the drain, so water would accumulate underneath. IMHO, ugly situation!
Have you considered building your own pan? It's not as hard as you think. An alternative to that would be Wedi or Kerdi. Then tile it. Epoxy grout is very resistent to mold and cleans easily. Check out www.johnbridge.com 's Tile Your World website for help on this as well...lots of ideas and lots of expert tilers there.