Home Network :: Airport Wireless Network?

I have a rather complex situation that requires (I think) a complex solution that I have not yet worked out! I have the following:

- ADSL internet connection

- TP-Link TD-W8960N 300Mbps Wireless N ADSL2+ Modem Router

- Brand new Airport extreme (n)

- Four brand airport expresses (n)

- 24 port 3com switch.

My requirements are:

- A contained, protected, office network with wi-fi with access, and also access for wired clients via the 24 port switch.

- A "guest" network with only internet access which needs to be spread throughout a large, old building (thick stone walls!), but cabling is not possible.

My inital thought was to use the TP-Link as only a modem and run the network via the extreme. Plug the switch into the extreme which would also run the office wi-fi and then extend the guest network (nice extra option in the extreme!) thoughout the building with the expresses. Problem is that you cannot extend the guest network with either n or g. I cannot just reverse these two networks because I need wired access to the office network.The current system has the extreme running only the office network (wi-fi + wired) as a "double NAT" inside the "mother network" which is DHCPed by the TP-LINK with its wireless on (pretty good range). I set the expresses to extend the TP-Link's wireless by old fashioned WDS which I got working but it was incredibly slow just because that is what g WDS is like. I am not using the n capacity of the expresses.

I thought about using the TP-Link DHCP and wi-fi to run the office network and then I can use the extreme and the "extend my network" n option for better speeds around the building, but then the office network would be open to the guest network.