Both red bay and green bay (normal) snooks bring a fairly high price and are in more demand than current supplys can support. They are reasonably simple to breed under the right conditions and can be sold and shipped at a relitivly small size. They fit your eventual size requirements and are in my humble opinion a great addition to almost any aquarium.
Breeding for profit is risky at best and I belive it would be better to branch out a bit. Start up costs can be controled by buying baby fish and then growing them out to form breeding pairs and slowly building an efficiant tank and feeding system to support the program. All of that takes time and patiance however. The changing nature of the market precludes breeding any one fish unless it's an industry staple like Angelfish...discus...Koi...ECT.
Freshwater clams...giant pond and apple snails...and crayfish all ship well and have a fairly good profit margin. They also have a well established demand.
High doller fish like freshwater rays bring a huge price but can be difficult to move on a consistant basis and, of course, have a preportionally higher start up cost.
What you start out breedng NOW may well be a fish that has saturated the market by the time you have young fish for sale so you really need to anticipate the future market carefully. IMO it's unrealistic to set a monthly income goal in ANY kind of breeding operation because you may have one or two MONSTER income months followed by several months of negitive cash flow.
ONE shipping disaster, even one thats not your fault can wipe out 2 or three months of profit. You also need to stay away from any fish thats in good local supply because you can't possibly compete with the huge numbers cranked out by giant comercial fish farms.
Most LFS want to be able to pick up the phone...place an order for a fast moving fish and have it arrive in time for an advertised sale.
You have a good idea and it's one that a lot of us in the hobby think about from time to time but it's VERY hard to pull off for all of the reasons I listed.
The best way to start a small scale breeding operation is to stick with what you know and don't get in over your head. In other words don't go into it EXPECTING to make money...at least at first.
It CAN be done and IS bieng done but once you turn a hobby or passion into a buissness you run the risk of losing all of the joy and pleasure you got into the hobby for in the first place.