I've set up my ar .223 to hunt with and my shots will be well within 200yards. I already have a spare Nikon 4-12x40 BDC. Would this be a good choice? It seems too much to me. Also what would be a good reasonable set of scope mounts. I had in mind the Nikon m223 mount. Thanks

That scope will be fine...

Mount...meh, just don't get bargain bin stuff and you will be fine.  That nikon mount will serve you well.

I agree, the scope and mounts will be fine.  

Yes, that's a lot of magnfication on the high end for hunting, but nothing says you can't just leave your scope set at 4X, and maybe increase it a little for a true 200 yard shot.  Remember, at one time 4X was considered a fairly signficant amount of magnification for hunting purposes.  

I have a 3-9X scope mounted on one of my 12 gauge slug guns that I don't think has ever been adjusted up from 3X in the field.  I dial it up to about 6X when sighting in at 100 yards, mostly to shoot the best groups possible.  I'd be better off with a 2-7X, but I just haven't gotten around to putting on a new scope.

For a little over a year (deer and coyote) season, I have had a 3-9x40 on my AR.  As soon as my TPS rings come in from back order, I am going back to my 2-6x32.

Everyone has different preferences, and more importantly, different vision.  Some eyes need more mag, some guys prefer more mag.  

Either way, learn to shoot with it and you will be fine.

ETA:  Good risers, good prices.  https://www.primaryarms.com/Primary_Arms_Mounts_s/1676.htm

Keep it on 4x and kill deer.  I've shot around a dozen with a Weaver Grand Slam fixed 4.75x scope (on a .270).  Mostly inside 125 yards due to the woods I hunt.