Hi @IanB
I like cedar for this roll as it is easy to work with and light weight and is of that houses period.
But rare to get and expensive so I would be taping into recycled timber places and recycling house shops and you be pick up bargins in gumtree.
As you are recycling and remaking the frames it dosnt matter if you get an old half damaged piece of wood you can trim the good bits out of it.
I would also contact building firms in your area and get chummy with them as they are the ones that do the house dismanteling before house rebuilding so you can get in on the ground floor before the massive prices at recycled wood and house parts shops.
We used to go into country areas and some times buy loads of bricks as we needed and cleaning up their site they almost paid us to do it.
Consider the timber in old furniture like old church pews and bed frames it might be just what you want.
The windows where of reasonably uniform size back then you might be able to pick up a few frames intact
There are other timber types too.
But if you are painting them just use pine sealed with paint with the old fashion router mouldings would be the way to go.
Have fun in you project.
