Hello! Feel free to take what I say with a grain of salt. I have no experience with chickens yet, but I'm also shopping for a prefab coop to start a flock with. I originally was also looking at the Sentinal, but I've since switched to considering the Zylina coops. Better ventilation, but I'm also considerably farther south and horribly hot and humid most of the year. The Nesteras remind me of those outdoor smokers, wouldn't feel comfortable putting my birds in one of those!
Plastic coops, I hear, are resistant to red mites though, so if that's an issue in your area that's a definite perk.
You mentioned being on a budget and considering a small flock of small to medium birds (like me!) so I'd thought I'd share the one I was considering. You can find it at TSC, but it's currently on sale at Walmart:
https://www.walmart.com/ip/seort/307110881
Good luck with your new fluffy flock!