Sounds like quite a project!
Due to the network layout using repeaters, etc you might need to compromise and expect a little lag.
Repeaters also mean you get half of the total network speed, so use 802.11g (54Mbps) for best results. Remember that with one AP per floor, everyone on that floor shares a single 54Mbps connection, so with a large number of users don't expect blazing performance for doing heavy file transfers. Repeaters will slice into this even harder as the data needs to be resent for each client on a repeater.
If you are using WRT54Gs (good choice BTW!) and can only get one cable between floors, why not run cables accross each floor and hook up three units as cabled APs using the built-in switch? The same switch could also daisy-chain each floor and then you have used all four ports for a good use.
That way the load gets spread much better and you could use the 1,6,11 channel rule on each floor (SSID can still be the same though) to minimise interference. It also means if you lose a single AP there is less risk of it dropping all the clients on one floor.
Roaming seems to depend greatly on clients as well as access points. The Intel wireless cards used in Centrino notebooks seem to do it very well (and allow the roaming behaviour to be configured) while some generic chipsets really struggle.
With such a big project you are going to have to experiment to see what works best. I certainly think you are heading on the right track though!