Thanks JB and S2 for the info. I was on the Kelvedon site but missed the windows apparently.
S1 glass windows are definitely interesting, we can get some pretty strong sun here and I worry about polycarb windows clouding quickly.
I have some ideas for windows seals, the frontrunner of which is applying simple U shaped weather-stripping to both the window and the frame, then using a flexible RTV to bond the two pieces together. Basically making a custom weather-stripping profile by bonding. Of course, could also just bond the windows right to the door if the cutouts match up nicely, I have some experience doing that from work.