She's gonna need at least 12 cabinets with two 18" drivers each, and probably around 15,000 watts of power. It takes some serious juice to get those those old arthritic ladies off their wheelchairs...
...so's tehy can WALKS AGAIN!!! PRAISE JEEBUS!
Ok, seriously, plenty of churches with bands run pretty serious equipment these days, but this will largely depend on the size of the room, the amount of kick they really want (depends on age demographic of congregation and musical style), and their budget. They will need an amp and a speaker cabinet, and maybe a dedicated crossover, too, although if they have a nice mixing board with parametric EQ they might be able to get by using that to shape the frequencies they send to the subwoofer's amp. Most PA subwoofers have 1 or 2 15" or 18" speakers in them. Power handling and sound quality depend on price.
If the room isn't too huge, they could probably substantially improve things themselves for $1000 to $2000. If we're talking about a big church with a big budget, it would be worth it to hire a pro audio company to help with the process and/or do the installation.