Some people hit the ground running when it comes to longarm quilting others take longer to get comfortable. It does take a bit of practice to learn how to load the quilt and how to handle problems. Tension is fussier but once you learn how to tweak it, it just becomes part of the routine. Some people are faster than others. But as it is with everything, it takes practice to get good and proficient. If your group does many charity quilts it might not be a bad investment. Maybe a couple of members will take to longarming like a duck takes to water and in addition to quilting for the charity can maybe try to recoup the cost of the machine by quilting member quilts for a nominal fee.
I absolutely love longarm quilting and you never know, maybe there are members in your church group who will also love doing it and the idea may not be so unfeasible.