I have a brad nailer, finish nailer and crown stapler. I use the brad nailer for woodworking projects. It's the most heavily used of the three. I use the finish nailer for nailing baseboard. It's also good for light "framing". I haven't done any crown molding, but I suspect I'd use the crown stapler for that. The stapler is also useful for nailing thin (1/8") board, e.g. as the back for a small cabinet.
As to what to use each gun for, the finish nailer uses bigger and longer nails. You'd be hard pressed to shoot a brad through baseboard and drywall into the stud. Conversely, you don't want to use finish nails when tacking a face frame to a bookcase.
For woodworking, you may also consider getting a pin nailer. It fires tiny nails that are almost invisible.
You don't need much power to drive these nail guns. Just about any 2HP unit will work (and for most brad nailing, even a dinky 1HP is OK). A floor nailer requires a bit more oomph. I suspect a 2HP unit would work, but you'd have to go slow. You might be better off renting something larger when doing flooring.
There are often deals on combo packages - compressor + nailers + hoses etc. I'd start looking for one of those at Home Depot, Lowe's etc.