I love the look of repurposed windows/doors for greenhouses, and I’ve even seen them advertised for that purpose at resale shops, but it’s really important to be careful about lead paint if you want to do that. Lead was the primary white pigment for a long time, and since windows/doors are often trimmed white, if they are older than 1978 (in the US, EU was 2003, though many member states had their own laws previously), it could be lead. Lead testers are fairly cheap if you want to go this route.