The key problem with being a wireless ISP is that when you have lots of users running at the same time even at low data rates you get all kinds of overlap in that data transmissions. This causes huge degregations in the system for everyone.

You best bet is to use a system designed to provide this service. One of the more popular ones is called canopy by motorola. Even though it operates on the same frequencies as wireless it does not use 802.11 protocols. The radio off the main tower actually controls all the remote radios so they only can transmit when they are authorized to. You get much better sharing and less waste of the bandwidth. Since this system is controlled end to end there are all kinds of QoS and bandwidth guarantee options you can do.

Now if your goal is to then provide free wireless at the far location you could hook pretty much any AP to the back of client radios and let users connect. This would still provide much better service than trying to share bandwidth with the main tower using normal 802.11 standards.