In your circumstances, a special resolution by-law may be the best way forward, but you’d have to include certain provisions.
1. That ALL owners agreed and signed up to it.
2. That this would apply to the lots into the future, regardless of the terms of sale of a property.
3. That the owners would agree to keep the windows in good repair, both visually and physically.
4. That individual owners would not change or replace the windows without the prior agreement of the other lot owners – this is to maintain uniformity of appearance. Fashions in window designs (especially prefabricated windows) change as different materials are used. You don’t want the the face of your building to look cock-eyed from outside.
Now, having said that, you’re probably going to need a lawyer to draft your by-law and that’s another expense. Sooner or later you are going to have to raid the biscuit tin and it might be worth working out what the total costs per unit are going to be in the various scenarios, then sitting down and talking it through.
In the worst case, you might end up taking yourselves to NCAT to force a special levy, and you want to avoid that if you possibly can.