Second hand gen2+ / gen3 that you mount behind the scope. I've had good results with this, but it really depends on how well the scope and mono combines.
Upside is that you have good optics (your scope) with zoom and you retain your original zero.
Downside is that it's heavy (but not really heavier than a NV rifle scope tbh), sits further back towards your eye (so your shooting position might be cramped) and it can be quite some faff to mount the scope correctly.
But i think that's the best performance for the money.
Check out the pic of my VSR with NV scope on. It's a cheap PST2000 scope mono that hovers around 600-1000euro. The mount could have been made better, it was for testing, but the performance was amazing. It was possible to track BB's to 100m through it (with some light).
You should also t hink about how you want to use NV.
You should always strive to use it as passively as possible, because other people with NV can see any IR light you make, and even normal people can see the red glow from a IR light.
If you want, get an adjustable IR laser illuminator instead of a normal IR light. That's better for sniping and gives you away less.
But always play so you do not need any light. Make your kill zones in areas with moon light or against a horizon.
It's very hard to see people inside windows with NV as well.