The first thing you'd wanna ask is whether the user (your pastor in this case) foresees himself needing to use the internet outdoors - specifically outside of a WiFi zone. This will narrow down the choice to between WiFi only/WiFi + 4G
Secondly, the storage size (16/32/64GB) needed largely depends on the kind of files he might be storing on the iPad. If we're talking about couple of pictures or songs, then 16GB is mostly enough; you might have to recycle the files sometimes but that's really just a minor inconvenience. More importantly, if he plans to say watch TV dramas or downloaded movies on the iPad, then you should definitely look at 32 or 64GB since a HD movie file can easily hit 3-4GB.
You never know what he might find useful, so if at all possible, go with the 64 g size. The extra hundred will pay off before you know it. As respects coverage, I am a wifi pad guy, and if I need cell coverage I use my hot spot on the phone. But in this case, the devise will likely find it's way into semi professional use, and it may make sense to get a 3/4 g version over the wifi only.
Pick your provider based on who has the better coverage in your area. The last thing he needs is a 'no signal' error while trying to look up something important at a sick parishioners house.
You'll eat through 16 GB in short order. Best to go with either the 32 or 64 GB depending on what he wants to use it for. If he's a big movie person and on the go a lot then the 64 is the best bet. If he's not doing a lot of traveling around and has ready access to WiFi then the 32 would suffice since iTunes allows you to re-download previously purchased movies (Movies tend to take up several GB of memory). I have around 100 songs (about 2 GB worth), games (about 4 GB), but my movies eat up a bit more.
So, I tend to leave them off my device since I have an Apple TV and can stream them whenever I choose and only load the ones I want to watch if traveling.
For the carrier, I'd check to see what the signal and speeds are in your area and make my decision from there.
You might also want to think about getting a WiFi only iPad. It costs almost the same to buy a 32GB WiFi + 4G/LTE iPad than a 64GB WiFi only model, so I would evaluate that. Also, he would have to pay for the 4G service and might not need it. Most places have open WiFi networks and if he has a smartphone, he could just use the HotSpot option to give his iPad internet access.
I'd go for 32GB of storage space because it is in the middle and will meet the average user's needs. 16GB might be too little space, whereas with 64GB, he might not even use half of the storage space.
I used to have a 16GB, which had less than 1GB left of space before I upgraded to a 32GB. The 16GB had my music (little under 1500 songs), around 300 pictures and the rest was filled with apps and files (books mostly). The new iPad (32GB) still has 10GB of space so I don't have to worry if I want to download a few pictures or new apps.