From my experience:
Sounds like a good idea, but before you begin, have a web site in place, at least.
In the 1960s and 1970s there were groups of serious and well-known photographers who did things together--had exhibitions, published quarterlies, etc. My opinion is that to have any credibility at all, any co-op needs to be much more than just a group organized to get cheaper ad rates.
Done right, and doing it right is the only way to do it, a co-op will entail a more serious financial committment than most of you are probably aware.
Someone, and that is some ONE, will have to take charge and make decisions--who is in, who is out. These things CANNOT be run by committee or it will not happen or will quickly fizzle out. This person should be paid for his/her time and effort, which will not be inconsiderable.
Everyone involved needs to have along-term committment. Years.
Ad is Black and White: Don had the story almost right. We met the very reluctant collector and had to contact him about four times. and even then it took two showings of our photographs before he committed to buy work. Only a year later did he tell us that he even consented to meet with us because he had been following our ads and saw that we were serious. The real point of this story is that to sell him work, we had to meet with him personally.
Do not expect to sell work, ever, from an ad in B&W. Not that it can't happen. It can. (Over the course of a couple of years one fellow sold 34 prints from one picture that he kept putting in one of the small ads at the back. I think the print sold for only $200, however.)
Which brings up price point. A lot more prints will sell for $200 than for $1,000. About 20-1, or more. but do you want to sell prints for only $200? There are pros and cons to doing that. It needs to be considered. Will everyone in the co-op have the same price on their prints? Probably not, this being an individual thing. But it is something to think about. My own recommendation would be that everyone in the co-op should be roughly in the same range. If you are selling your prints for $1,000, you don't want to have others selling theirs for $200--at least not in the same ad.
Again, do not expect to sell work from an ad in B&W. If that is your expectation, you will be disappointed and the energy for the co-op will soon dissipate. An ad in B&W must be looked on as only a part of a marketing effort, not the whole thing.
Relationship between having a feature and advertising in B&W. There is no relationship, really. One reason there sometimes seems to be is that by advertising, the Editor/Publisher, Henry Rasmussen, who, by the way is wonderful to deal with, gets to know your work and will ask if you would like to have a feature. Recently, we recommended a photographer to him, whose work will be featured in a forthcoming issue, and this photographer cannot begin to afford an ad of any type. And often, we know of at least several cases, where a feature was offered and then the photographer thought to begin advertising. Henry wanted to do a feature on us for years. He thought we were a good story, being a couple, yet still quite different. We wanted to wait until the magazine became more established, which is why this did not appear sooner.
There is so much more I could write, but this should do for starters. Bottom line: good idea theoretically, practically there are lots of pitfalls and problems, ahead, as well as a lot of harder work and effort than you can imagine if you want it to be successful and not just a lark.