I don't know how to choose an adequate antenna
What you are looking for is to build a "link budget". Here is a calculator. First consider the transmit strength and receiver sensitivity of your system. Then figure out all your losses. If your transmit strength + losses > receive sensitivity, then the system should work.

You will have at least 111dB of free space loss. calculator. 
I would also budget something like 30dB for margin due to multi-path loss, occlusion ect. Weather could become a big factor and stop your system from working as well if it's all outside.

Now your receiver antenna 14dBi of gain, so assuming it's aimed correctly you would need a receiver that is at least -127dBm sensitive (-111dB - 30dB + 14dB). If you have a transmit antenna with gain, or a transmitter with power above 0dBm then you can relax that requirement. 

Typically RF settings are already in dB for transmit strength, but in general the relationship is the log relationship for power. A doubling of power will increase signal strength by 3dB. A 10x power increase, gets you a 10dB increase in power etc.

To design the system you have to pick you transmitter, receiver, transmitter antenna and receive antenna at the same time. Picking one without the other doesn't make much sense.

This paper may be a good read as well.

Also dBi, dBm, dB etc can get confusing so this is a worthwhile read too.