I don't know much about Drones but I know a bit about IR, best results are from a cooled IR camera, so that adds weight and power consumption, a cheap uncooled IR camera may not have the definition the OP is looking for. Presumably he is seeking a temp differential in an area of land compared to...
It is good when people make recommendations
I have no knowledge of these things .
Just a question, the OP needs thermal imaging ( IR) Does your platform support it?
Basically anything with a camera ( from what I have read on that) has regulations but equally those regs prevent using Drones in built up areas so may be of limited use to you.
Before selecting a Drone, check out licensing regulations, anything beyond a toy may need one. I would guess infra red would require a decent camera therefore more likely to be regulated
You may need an Owners and a operator license and CAA approved training. Also you need to consider...