C2s are great beginner thermal cameras - their resolution isn't the highest, and you must get fairly close to the surfaces to get good images, but they are a great way to introduce the technology into your protocols without taking out a loan, heh. They have the look and feel of typical point-and-shoot digital cameras and one can be proficient with them in a matter of minutes. They're also great for letting a panicked, upset customer or penny-pinching adjuster feel like they're a part of the diagnostic process; I have seen customers actually laugh and smile when handed a thermal camera and told to "take a peek around", despite the fact that their sofa is still standing in water. Not only is it a good distraction from their worry, but they now feel involved, and less like they're going to be taken advantage of, since they have visible proof of where the problems lie.
The "stick" type E-series cameras - E4, E6, E8, etc - offer better resolution in most cases, plus more functions in the unit. They're a great mid-step camera, or even a beginning camera for someone who's tech-proficient. (This is the type of camera used by the "ghost hunters" in the TV show American Horror Story: Roanoke, by the way)
Once you get comfortable with thermal imaging and its limitations (remember, you're not seeing moisture, only temperature differences that might indicate moisture), you can progress to more advanced models based on your needs. Some offer report-building and WiFi capability, video recording, data logging, they even have combo units that act as moisture meters, psychrometers, and cameras all at once. These can be less intuitive to use and are best bought in conjunction with a training class to really get the most out of them. I don't usually recommend these as a first IR camera, but rather as what a lead tech or owner would get once he passes his old C2 or E6 along to the next guy down the pipeline.