SICK HEN! Help is needed!

Hello,
I have a Bantam Rhode Island Red that is ill. About 3 / 4 days ago she was the queen of the flock (of 3 in total), running around my backyard, eating worms, digging around generally having fun!
Then 2 mornings ago I found her hunched up, feathers fluffed, neck in, not moving about etc. As she is not a very friendly hen I couldn't catch her to see the problem. I left her for a bit. For 2 / 3 days she stayed like this and I also noticed her not eating or drinking, socialising and looked very listless and "staring into space". Also she hasn't been giving me eggs for about a week.
I have since cleaned out the cage, nesting area and sprinkled Pestene around and changed their drinking water and added a solution for killing worms etc.
I have inspected the sick hen and forced her to drink some water with the solution. Also broke an egg and whisked it for her and gave her some bread soaked in milk and honey. She didn't really take an interest in this! (usually she is the 1st to eat!).
Last night she could not get up onto her perch and today I let her out into the yard and she ate some grass (which also had dew on it), her posture seemed fine and her stool is a little runny with not much 'body' but overall fine in colour. Then I noticed she is starting to have a wheeze.... almost like she starts to squawk and ends in a strangulated cough / splutter!!!!!!!!!!!
I have been only in the 'hen' business for 11 months. The 1st 2 hens I brought were from a so-called 'poultry breeder' which took his birds to shows etc. Unfortunately he did not tell me that he had issues in his flock with Marek's Disease. 2 months into having my 2 hens (at that time about 7 months old) one of them came down with Marek's Disease and we had to put it down.
I have since introduced 2 small bantams of similar age and they seem to be doing fine.
I am not into taking my hen to a vet as this can become very expensive!! Could you give me some advice on what my hen may have??
Please help!

Thanks kindly.