Coughing and discharge from beak

Coccidiosis is treatable. It also spreads very easy to other birds and kills quite quickly too. 

You can add a treatment Coxoid to their drinking water which is very effective if caught early enough and is cheaper than a visit to the vets. Probably not much use to you now but if anyone else has a similar problem it may help them.

You will also need to clean down with Bio-oo-cyst to kill off the oocyst to prevent other birds picking up the illness.

Im not sure their only problem is coccidiosis, i dont think there is coughing, sneezing or beak discharge associated with it. If they are generally not in good health from one illness, then they would probably be susceptible to other illnesses which the antibiotics should treat.

I cant advise whether you should have the chickens replaced but if you do, make sure you have treated the area with Bio-oo-cyst, otherwise you will be back at square one.