I've done a couple of Churches now. Can use WFP carefully on stained glass, but make sure that the windows are watertight. I am so glad I did this one the first large window I did as the water went staight through on just a test run. Finished it with ladders, slow and laborious because of the leaded. So b4 you quote you had better have worked out what method you will use for the externals , or you could become well unstuck when costing/time taken are analysed.
I like the modern churches. Much quicker.
Read the previous posts on this subject because the guys who replied to me were very helpfull, about this topic, and on the cost and value of ancient stained glass windows. Would make your hair fall out.