I'm thinking it might be a useful diagnostic aid for finding hotspots in an engine bay, but before I drop 250 on a Lepton 3.5 I'd like to try one out.
You just want to build it for fun?
I have a FLIR one attachment for iPhone, tiny little thing I bought from the apple store. I bought it to check if my underfloor heating was working and it did the job just fine.
It would be used in a professional garage as a diagnostic aid. OBD/CAN, WBO2, a couple of voltage probes, an Amp/frequency display, endoscope. (Not all at the same time.)