Bruce, the type of optic you are describing will be very hard to get within your budget, especially if you are wanting to use it as a clip-on (in front of day optic).
You can find a PVS-14 gen 3 used in your price range, possibly some new, and they can be mounted behind a day scope (not in front like a clip-on unless you get a collimated lense setup that would also have to be mounted $$). You can also mount the PVS-14 behind red dot sights, and it is designed to use as a scanner/navigation.
If you truly want to do it right, get a gen3 night vision scope for the rifle, and find a thermal scanner. This would be a hunting setup, you do not want to navigate with a thermal monocular as you lose detail/depth perception and will walk into a lot of branches or step into holes. The NV on the gun is for positive identification, the thermal scanner will show you what is alive around you to look at with the NV scope/clip on.
The scan and mount to gun for 2-3K is the thing most people want but it does not really exist in a single unit, and if you cut corners to try to obtain it you will regret it down the road.
You can get a Photon XT or RT digital night vision scope and a FLIR PTS-233 Thermal scope within your budget. The thermal you can scan with or mount on several rifles quickly, get a good image and be multiuse for you. The digital night vision will just need an aftermarket IR illuminator and you will be able to see clearly pretty far, further than you might be able to shoot accurately.
The Flir 233 Thermal will run you 2k, the Photon XT/RT are 4-700, illuminators vary.
Also look into tripod setups if you do not have one, scanning with a NV/Thermal on a right is awful, unless you are hunting over a baitpile or have an area to hunt that isnt 360. Tripod setups can be very expensive as well, with the lower/mid range budget being 500$.
Lone Star Boars as mentioned above did a review on the Photon RT recently and it was very impressive for the price, I will be picking one of those up for a backup rifle/friends.
Several Reviews are out on the Flir PTS233, you can find several reviews online (Night Goggles has a good video on it)
Thanks