Most consumer routers don't support client/ad-hoc modes as the companies prefer to put those features in their "access point" products (which are usually the same hardware as the routers - just differentiated in software).
I've used DD-WRT (related to OpenWRT), which has an easy-to-use web interface, on some cheapy broadcom-based routers like the Linksys ones. It supports client and ad-hoc modes easily, so you can make links between units and see the stats/signal levels. It will turn a standard consumer "router" into a very powerful networking device, although you must remember that Broadcom radios aren't good RF-wise (low/bad sensitivity).
That software/radio side of things is pretty simple compared to the outdoor antenna mounting, grounding/insulation, and site survey aspects. I've been doing "professional" wireless setups in the countryside for a few years (up to 16 miles on 900mhz), so I can give you a few suggestions, or better pass by and do a site survey, if you want.
-C