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.