Agree, they'll be packed like sardines in 5sqft. When I first got chicks, I completely underestimated just how fast they grew and how active they are. Not to mention the poop. They're factories. I'm sure many owners can attest to that as well. Limited space will increase the likelihood of...
I'm generally against "pre fab" housing. They look nice and seem like they'd be useful but usually aren't. The advertised dimensions are, at best, 7sqft of space and around 5sqft with the "layer (internal)" dimensions. That's not a lot for 20 chicks. The more space they can have the better off...
I'd definitely go the outdoor brooder route since theres a few months to get things together. My first flock was brooded indoors a spare room and it was the worst idea I ever had, lol.