All of my present cats were born feral (3 of them) I joined a group like you are describing and for a few years I fostered feral kittens and socialized them.
If you decide to take these cats on permanently as ferals, you can make them a shelter for the winter time. They will stick together and stay warm. Most feral cat caretakers I know do not deflea their cats, but if one is reallly sick, they try to retrap and treat them. (or even put medicine in food which doesn't make sense when you're feeding more than one)
I started with ferals when I looked out my back window one day and saw kittens in my back yard. It took a month to actually trap them, but I did trap all of them and their mother; spayed the mother and let her free, and socialized all the by then 4 month old kittens. One of them was Enza - and she is one of the greatest cats I've ever had.