In the past, I've only used FLIR thermographic cameras, they give good pictures but are (a) bulky, size of a big SLR camera and (b) expensive, i.e thousands not hundreds of pounds. You'd need to be using one a lot for the extra income generated to offset the capital cost. You can hire them for a day/week but that ain't cheap either.
These days you can get a small thermal imaging sensor to plug into your smart phone for about 10% of the cost of the above. These are much smaller, which can be a big advantage, but the image quality is not going to be as good. If you want to dip your toe in the water, I'd suggest getting one of these (FLIR do one) and see how much use you get out of it and whether paying (a lot) more for higher resolution and sensitivity seems justified.
If all you have is the thermal image, it's not easy to interpret what you are looking at later, e.g. if you have to provide a report. The cameras I used allowed you to superimpose the thermal map on top of a visible light image, which made figuring out what was being shown a lot easier, pretty much like the ones in this post:
Find Is this a leak. Thermal image camera Advice and Help. How-to Is this a leak. Thermal image camera in the UK Plumbers Forums advice boards on PlumbersTalk.net - Plumbers, Gas Engineers, Bathroom Fitters - Free Advice. Is this a leak. Thermal image camera
www.plumberstalk.net
As that discussion illustrates, having the image is one thing, deciding what they mean is another.
I've always found them quite fun to play with so I doubt you'd regret getting an inexpensive sensor-only one to go with your phone. You can always flog it on eBay if I'm wrong.