When we need op amp?
Let's talk about the analog side of things for a minute, without worrying about the specific parts. 

If you have a voltage signal that you want to send down a long wire, whether or not it arrives unaltered depends on a number of factors. DC signals don't particularly suffer from any of the effects of transmission lines, so you are left with three primary factors - output impedance, input impedance, and the resistance of the line itself. 

In the event that input impedance is very high, and line resistance is very low, it is predictable that a voltage signal can travel quite a long ways before something needs to be done about it. At the end of a 50 meter line like yours, I'm not so sure we're in this situation.

So what, then do we do about that? We can't just arbitrarily amplify the signal, it has to be predictable. And it still may have to cross a long distance, which could cause even more voltage loss and signal error.

The best and most industry standard way to solve this problem is to convert the voltage signal into a current, near the original signal. DC current on the line will be exactly the same at all points regardless of voltage drop, provided the output can provide enough power to drive that current. Opamps are a convenient and easily designed method to build a voltage controlled current source, which can then be passed down the line to a receiver. This receiver can also be readily designed from opamps as a current controlled voltage source. By selecting matched parts on both ends, it is possible to transmit simple control signals very long distances with very little error. 

If you decide to use these methods, the gain on either end is almost entirely up to you. You could even decide to use the standard 4-20 mA method, which has the added convenience of making the system able to detect a disconnect and report it to the user.