Since money is not an object here, we will go all out
Over priced and not needed for just 500 users - plan:
You could pick this firewall
It's a SonicWall, with the power of having 750,000 simultaneous sessions at once. Able of pushing 5Gbs with an 8 port 10/100/1000 lan.
Now you could run STP (shielded twisted pair) ethernet cable to each floor with
router. Make sure that you alternate the channels of the routers per floor.
You could use the firewall as your connection point, but I personally would use a switch. This
would do just fine in this particular setup.
Setup that is not so dramatically over priced:
Here is a nice priced Cisco firewall: link
I would go with the same wireless router setup, same router. link
Here is a nice Cisco switch (10/100/1000 speeds): link
For all cables I would go with cat6 STP. (buy in bulk) Newegg doesn't seem to have in bulk but here is a place that does: link