Buy the camera with the highest resolution you can afford.
I know, this sounds intuitive.....but pixel count does increase cost rapidly, and thus one tends to choose lower priced models..
Heat tends to spread out and smear on a board. But if there is a particular SMT component which is running hotter than its surroundings, the higher resolution will help you spot it out.
Edit;
once that you have a FLIR camera, you'll find many useful and interesting stuff to do.
The one that surprised me the most, is how trees become aglow at night in the summer time. Tree leaves act as heatsinks which remove heat from the surroundings and radiate it back to space. I had known that LANDSAT and similar satellites use FLIR cameras to detect vegetation, but seeing it yourself is a sight to behold.