Mick,
I've got the N Vision Nox 35. It is the best scanner that I have ever personally used, but pricey. It has a reticle and rifle mount so it can be used as a scanner or a scope. The reticle can be disabled and the mount can be removed. It also has a removable dovetail mount for helmet mounting. IMO the 35mm is a little big for a helmet mount. They make the same unit in 18mm. That one is what most guys are using for a helmet mount. It is pretty much the same size as a PVS 14.
I would buy the Nox 35 again, no regrets after I got over the sting of the price tag.
I tried to go the helmet mount route and it did not work for me. The scanner got in the way when I tried to get down on the rifle scope if that makes sense. Ideally you would turn it off and flip it up on the helmet before you get on the scope but I just could not get all that done in time to get on a hard charging coyote. For spot and stalk or bait pile hunting I can see where it would be good though and if that was what I was doing I would go with the helmet mount. Some guys can make it work for calling but I just could not figure it out. Maybe if I was younger with better reflexes it would have worked.