If your co-op's central heating is very uneven, I'd look into that more closely. If some units roast while others are cold, maybe the fault is with how heat is distributed through the system. If so, that can be a problem no matter what new windows you install. A good engineer can make recommendations, including possibly installing individual heat level adjusters in each unit.
I agree that Pella windows are expensive. They seem to be "hot" right now, but if your co-op can't afford them, it would be financially imprudent to get them, and the extra cost can be better spent on something else. We have thermal barrier, aluminum windows with 7/8" insulated glass. They look nice, work well, reduce noise, and run $350-$400 per window. I believe you can get the baked enamel finish in white, black or copper color. We have the standard white.
If your co-op's 12 units all have the same old windows, you should replace them all. You can schedule the coldest units first, but it takes only a few days per unit. You can do them all fairly quickly if you plan ahead with the window vendor and shareholders. Budget for ALL the work. You can't do 4 units now and hope to do the rest later. What if you don't have money for the rest later? The project will drag out, shareholders will complain, and the building won't look very nice. Also, Business Corp Law requires that you treat all shareholders fairly and equally. You can't do something for some of them that you don't extend to all of them in a timely and equitable manner.
Just my two cents.
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