This won't be the most popular response, but I don't think it's AWFUL if you start out with a prefab -- as long as it's only a temporary solution, as you said it is. Some are better than others, and some can be modified to work while you build or get something better.
I suspect you already know that, based on your research. I know everyone wants to help you give your chicks the best possible lives without discouraging you or making you feel as if you aren't doing your best.
Honestly, my avatar coop is a prefab that has worked well with a few modifications. It's incredibly sturdy, needed extra predator protection over the windows and a bit of added ventilation. I got it off Craigslist at a steep discount when its previous owners needed more space for their growing flock.
I have two other prefabs -- one is an Urban Coop, better designed for the warmer winters where it's manufactured, and the other a little A-frame that houses two Bantam Cochins. Both have been "improved" to make them safer and better able to protect the girls from Iowa winters.
I also have "homemade" coops, and my poultry are all doing fine. I suspect yours will, too, because you care enough to make sure they have what they need.
Welcome to BYC! Hope your "roo" situation works out they way you want!