Are you going to want a stand alone optic, or one to use in conjunction with another optic?
Do you have, or will you use, an IR illimunator/laser in targeting?
What do you want to shoot? Does it shoot back? Does it have IR treated gear or it's own gear to look for IR illuminators?
Do you need or want to be able to see through smoke/fog/haze/camo?
I used to have a PVS-14, relatively late model, Gen III stuff. It offered a versatile range of options. You want one made by ITT, not ATN. Three or four grand. Or about four grand will get you into a surplus military issue thermal weapon's sight. (Really nice clip on TWS units can be had, but run about 21K...) I got a deal on my PVS, traded an IMBEL FAL clone and a bunch of mags for it. Later ended up selling it since I never used it much.
You can easily use a PVS-14 with an appropriate
Aimpoint or Eotech unit, with or without the magnifier. Or you can use it as a goggle (on use the ANPVS-7Deltas) worn from your headgear and in conjunction with an IR laser or illuminator.
These won't work in terms of being able to see through cover, smoke, etc. Nor will they work against camo - natural or man made. With their built in IR illuminators, they will work on totally dark nights, but that will reveal you to anyone or anything that can see IR .
I'm still working on getting my PAS-13 mounted (another member helped with a mount, still need the right screws). That's a Thermal sight, those are good night and day, though mine is just a two color unit and not one of the more colorful FLIR units. It's about the size of a Desert Storm era "starlight" scope and was state of the art circa late 2005, since outmoded by the much smaller/lighter clip on units. You can probably find one at Knob Creek. These see through fog, light cover, camo and work day or night equally well. The military units will have a built in aiming recticle and optional magnified zoom.
I got mine from a guy who had indeed bought it at Knob Creek. He used it to hunt Bigfoot. (Apparently Bigfoot can see in the IR spectrum... Shrug.)
You have to a U.S. citizen and agree not to export any of the good stuff. For the IR illuminators/lasers you'll need to either get FDA permission (they regulate lasers) or else look for a gray market item sold off as surplus.