To tune down, you want long-scale or heavy strings. Or both.

Apart from some very expensive classical guitars, steel string ones are louder. This will help to get your pastor or whatever you call him in time and in tune, and likewise the choir.

Custom strings for steel guitars can be obtained more easily than for classical.

BUT the lower you tune a guitar, relatively speaking the quieter it seems (a function of the ear's sensitivity curve).

The best answer is a steel strung guitar, a capo, and some practice.

When I was in the school choir (and unable to get out of it) not only was there choir practice for a whole evening a week, but also there was "congregational practice" in which the whole school was taught how to sing new hymns.

There is no miracle cure. The capo is the nearest. It doesn't take much figuring out where to position it since you know there are twelve semitones in an octave.

D'Addario offer some good charts for string tension, but they are for steel strings. There are also java applets that you can download.