Here’s a comparison chart for all the Nikon speedlights.
The SB-400 is very small, and takes only 2 AA batteries, so it would definitely be nice and portable. It won’t be as bright though for lighting up a large church or building. You could try using an SB-400 on-camera (bouncing off the ceiling) and a larger SB-600/700/800 as a slave for additional light…
Looks like there’s not a whole lot of difference in price used between the SB-700 and the SB-800… They would both give you a lot more power than the SB-400, and can be set to “slave” mode to be triggered from your camera flash.
I strongly recommend bouncing your speedlights off of a white ceiling or using reflectors/umbrellas to modify and diffuse the light. The first time I used umbrellas with speedlights I was just BLOWN AWAY by the quality of light you can get just with some simple diffusing! Of course you’ll need light stands to hold the flash and the umbrellas. I was fortunate to get some nice stands with all the accessories very cheap from Craigslist, but you can find all sorts online. Then you’ll also need something to mount your flash and hold your umbrella in front of it.