Too much to consider.
Zoning, will it be on city/county water & sewer lines or well & septic tank.
Will it have enough bed space and handicapped accessible bathrooms & showers.
Is there at least 1 very large room for retreat attendees and another for just plain relaxing or doing handwork.
Will kitchen be big enough for more than 1 cook, with a very good sized refrig, and maybe even a freezer.
Is dining are going to be big enough.
Who will do the daily maintenance, cooking, etc.
Are the outdoor views enticing, with maybe even a porch.
Will you make it 'scent free' as many have respiratory problems or get migraines for various scents.
Will there be walking trails and places to go that are nearby - exp a quilt store.
How safe is the neighborhood?
Most of the rest is already mentioned. I know in our are the group that attends is required to have insurance on their own or pay for it through the retreat center.