And other little unimportant things like your Budget.
If you use WiFi Routers you can not get the computers in Building 2 to See and Share on the LAN in Building one if this is required, you would need to turn off the Router Function part of the Device so it becomes a WiFi Access Point.
I wouldn't recommend using Range Extenders which may work but they would also allow others within range to connect and maybe gain access to the Data on the LAN. I've seen some advertised with a range of up to 850 Meters which is a very large area to have to secure.
You also have to look at the number of Floors involved as you will very likely need 1 WiFi Access Point per Floor and depending on the construction of the building very likely more than one per floor to get sufficient coverage. Also depending on the internal Wiring you may be required to either run some Cable or risk the use of WiFi Extenders so you would need to chose the hardware carefully to minimize any chance of outside Unauthorized Access to the WiFi LAN.
Col