I have actually done a platform or stage like this before. Another decision, do you mind the platform? If not it is a good compromise. Make sure that the drain has a p-trap in it. Without a p-trap sewer gases will be released into the bathroom, dangerous and stinky (methane I believe). The stage should be very sturdy, probably made from 2x6's and 3/4 plywood, minimum. The sturdier the better, you don't want too much movement, it can cause the fiberglass base to crack or cause proplems where the walls meet the base.
Some shower bases require you install them on a bed of wet concrete product of some kind, but most shower bases these days have supports to allow them to be put directly onto a plywood floor. Check out the base, look for ribs or what looks like a good structural support under the base, some are better then others and cost the same money.
When you go to install the base make sure you measure, and then measure again. Dry fit you plumbing and base before glueing or finalising your installation. The drain should fall as close to dead center within the hole in the base as possible. And if you are using PVC plastic pipe make absolutly sure there is no weight on the plastic Pipe, it shouldn't be under any pressure (in other words don't use it to prop up the base, and be careful about shimming under the pipe, instead strap it into place) if there is pressure on the pipe it can crack it. Nothing sucks worse then when you are in the shower and hear a loud popping noise and then water starts going everywhere. this happens because the base will flex with the weight of a body.
Running supply lines under the stage is alright, access will be limited so test the lines before covering anything up. Make sure not to pierce with nails or screws :-) that's fun too.
Also if you use a stage, pay attention to details, like how am I going to finish this stage. Should the front of the shower base be flush with the stage or should it extend out and be like a traditional step, if so how will I finish it. I can tell you that flush looks awkward (trust me) the bases always have a slight angle to them. Even a small lip looks better. make these decisions before you build.
One more thing, if you are going to install a shower door or curtain rod, put something in the walls to screw to. So later on you don't have to reley on plastic wall achors to attach a curtain rod or shower door.
PS Two most popular pre fabed bases are made from Acrylic (more expensive) and Fiberglass. Advantage to acrylic, easier to keep clean, looks better in general. Fiberglass is cheaper, tough and will stay clean if you stay on top of it (but it will stain)