I know it's for 'security', but what's your actual application?
Thermal security cameras exist as commercial products, usually in pan and tilt turrets, and you can get them on the used market - probably the simplest option. The moving turret helps make up for the lack of spatial resolution by letting you choose where you point. Making your own is a possibility, and while changing lenses really isn't a huge issue so long as you have the same mounting threads, it's difficult enough/expensive enough to find individual lenses that it's probably going to be cheapest just to find the lens you want on the core you want.
But again, the application is important. You don't get much resolution even with a high resolution core and a narrow field of view lens, and even if you did, a recording of someone's face in thermal wouldn't go far for recognizing them or reporting it to the police. A thermal camera will see heat in pitch black no problem, so a wide angle lens can be great for object detection over a wide area, but even with a 1024x768 core (some of the highest resolution available in the five digit priceranges), a wide angle lens is going to leave your complete target spotted at just a few pixels wide and nearly impossible to recognize. If you actually want detail on your subject, you need a higher focal length lens and a smaller field of view, which would mean a best case for a thermal camera used for recognition of a subject would be looking down a long hallway or similar, but again, actually getting identifying information from your image when no one has thermal vision is tricky.
I think the case where thermal cameras get used for security is for dark areas with staff on site. It's not that valuable to have a recording of a thermal image after the fact, but if you have a security team that can spot a subject on a thermal camera and then go investigate, it could actually be a useful tool, and you see thermal cameras used for search operations (land, air, and sea) for this very reason. My guess is such an operation would have a fair bit of budget to spare, though.