I want multiple Arduinos nano connected to an RFs/WiFis modules to read temperatures and send them to a central Arduino connected to an RF/Wifi module... (many routers to one coordinator).
What choices do I have? I need low cost RF/WIFI modules (up to 3-4 euros/wireless module) not Xbees. I would prefer a transmit range 800m - 1000m (and more if it is possible).
The specified wifi range for typical applications is about 300ft (~100m) although you can extend for a multiple of that when there is no obstacles and no case covering the board (a good reason for choosing modules with external antennas). If you make use of Wifi extenders/routers with high signal strengh, this helps of course.
The ESP8266 as Easyrider83 pointed out is a nice choice in the sense of being a standalone solution, what means, this board can make the whole job itself (communication+measuring), whereas an Xbee - which seems more suited for larger ranges - acts as a peripheral device, needing a local CPU to handle it.