I have played with this at one of my previous companies before i sold it off the offices were about 100+ meters apart, we has at the time WRT54G-V2 Routers, so we flashed them and installed Open-WRT on then, we did this so we could install Asterisk on them as well (Yeah i know a little Geeky but hey it was fun).
I am not sure of your Linksys Model but the WRT54G with its original firmware supported QoS a very basic form of it, but it still supported it, Open-WRT had some improvements on it that gave slightly more options then the Original Linksys firmware.
I am not sure why you would want to purchase Asterisk from one of the local Stores, not that i want to deprive any business of money, but since your a Not-For-profit organization why don’t you install Open-WRT (I think you already have) and then install Asterisk on them, Your version of the router has way more processing power over the ones we were using so it can handle quite a few calls concurrently using the G.711a/u/LAW CoDec’s or even even some of the other ones that don’t require huge amounts of processing power or Bandwidth.
To take it one step further, with Open-WRT you can even safley overlock the process in the router, the system will tell you if it wont handle it through the SSH session into the router, it will tell you if you have gone too far, I.E yours i think is either a 200Mhz or 250Mhz Processor, you should be able to safely over clock it too 300Mhz.
However with that solution there is a downside, you cant store voice mail messages, you will have to work out a way to forward that part of it to another machine some where else on your network. but doing it this way you have End-2-End control over the voice traffic as you have Asterisk residing on both ends.
But i caution you on this level, you will not get full 54Mbit over your the Wireless link, especially over that distance using WEP/WPA, even if you have an 18DB Antenna on both ends it will still struggle a little bit due to Air traffic that will be passing by (Also Known as Air wave junk), so no amount of QoS you do over the Wireless link is going to avoid Packet Loss, you can only try to minimize its effects via Jitter buffer and the highest possible quality CoDec you can use. Then i add on top of that, over the Wireless link you have to keep in mind that the link will hit Saturation point at about 32Mbits (Assuming you can even truly get that which i highly doubt it), once the link hits Saturation point your going to start to loose a lot of packets and no amount of Jitter Buffering you do, or even the best quality CoDec you use is going to resolve that problem for you.
You are going to have to plan this out very very carefully, work out how many calls you expect to move over the link, preferably at the same time, what CoDec are you comfortable using (Obviously the higher Bandwidth ones at the better ones), and what are you plans for future scalability.
My only fear is you will blow the 12K and end up finding out you have a network that can not go past the point it is at now, do not let any sales person pull the wool over your eyes, i see it happen all the time, sales people are good with the technical jargon and are happy to take your money, but not there when you need them after the sale is done.
I hope you make the right decisions for this project of yours.