To start with you would only need to get 1000 m range of the RS-485. That is of course if you put the main controller in the middle of the 2000 m fence :-)

And with a low baudrate you can get up to 4km I think... so this should not be an issue.

When it comes to IR led a normal TSAL6100 can be modulated with i.e 40kHz to get a carrier that fits a normal TSOP4140. You can of course have any carrier fq but 33k 40k and 57k are common. Then you send data on this carrier in short burst. Short enhough to be able to drive the led really hard 500mA or something and short enough so you don't overheat it.

A good guess would be that intruders are more common during night time than daytime and so getting 50m is not that hard. After 50m there is a receiver and a new LED and so on.

If you would like to try a normal magnifying glass attached at the end of a pipe is a good start. To get 50 m you would need a 25-50mm diameter and a focal length of atleast 150mm to make it work during daytime.

If you need more info you can PM me