It's rather simple. You have your high speed DSL/CABLE to some router, then connect WAPs (you'll know the acronyms by now) at each floor. A broom closet is not unusual.

Outside of wiring, the cost should be very cheap.

I'd alternate between channel 1 and the highest channel as I ascended each floor.

These are commodity items so unless you are in need of something special, a trip to best buy, newegg.com or compgeels.com would fetch the hardware.

Bob