i bought two ubiquiti M5s from freetv,ie, going strong for 3 years. im not using them to broadcast wifi just to transmit, think you can enable them to broadcast Wifi as well. M5s wouldn't be suitable as they broadcast at 5gz and i think you want a version that broadcasts at 2.4ghz.
its quite simple
your router has a name or in technical terms an IP address 192.168.x.yyy
transmitter 1 is plugged into the router using a LAN cable, mounted outdoor looking at the target location. transmitter 1 has a name 192.168.x.zzz
transmitter 2 is in the line of sight of transmitter 1, transmitter 2 has a name 192.168.x.vvv. If wifi broadcast is not enabled on transmitter 2 then you need to plug it into another router to broadcast wifi.
x needs to be the same on all 3. When you get them you have to preconfigure each to the formats i have listed above, via plugging directly into your computer.
yyy, zzz, vvv need to be numbers between 1 and 264 i think is the max value.
if you are transmitting a distance of 100m, then you need a transmitter set that is rated at 200m, thats what i found anyway.