Resolution is important. Take a look at the
12 days of thermal riddles to see how confusing low resolution images can be. (Those images are deliberately made to be difficult to interpret).
Also have a look at the
Thermal Imaging Gallery where you'll see sample images from a wide range of devices and resolutions.
If you need to measure the actual temperature of injectors and exhausts then you will need a higher quality (radiometric) camera. If all you need to do is see relative hot spots (for example, 'this injector is hotter than the other three') then the majority of thermal cameras will do.
Personally, I would avoid resolution lower than 160x120, for ease of interpretation; 320x240 (or 384x288), whilst significantly more expensive, is in my opinion generally more revealing.
Adjustable focus will be a must if you intend to use the same device for PCBs and engines.
It may be worth your while buying a used, low resolution Seek or Flir camera on eBay or similar so you can get the feel of things, then buy a 'proper' camera and sell on the lower resolution unit. You're likely to get more or less what you paid for it.