Don't misunderstand me, 29ers have their place, and your terrain is really probably ideal because even if you became a "real hardcore off road rider" I doubt you would face the kind of technical terrain I live for, and even at my age (well into my 50's) I daresay I could challenge any rider on a 29 inch bike in slower technical terrain and hill climbs and more often than not eat his lunch, not because I'm so much better but because geometry would be in my favor. On a seriously fast (very seriously) downhill I would stand no chance, but it isn't what I necessarily ride so it is a moot point.
In ruts and snow I think you would be well pleased with the way it would soak up the hits but I can't imagine going full rigid, a hard tail is the better compromise because it gives you some flexibility off road, and no way if I am going off road even a little bit would I ever opt for a single speed, no worse choice could i imagine.
Again, it depends on what your willing to spend. Maybe I am just still old-school but 26 inch bikes offer more overall flexibility and of course i have always been a pretty big Cannondale fan have owned and ridden at least 5 of them. The Trail series bikes are well designed for what I think you're looking for and there are 5-6 different price levels depending on the gruppo and they are available 29 and 26 inch. The C'dale F series bikes are still plentiful on Ebay and as the frames are always the same, it really boils down to what you're prepared to spend, and components are where the real money comes in, buy a used bike on Ebay with a good component group and you will not be disappointed.
Since your post I have read up a little more on belt drives and I think you might make a mistake there, belts are prone to early wear and once they go they are useless, not so a chain, a chain will take a whole lot of abuse and keep on going in cold wet weather just keep it lubed at least every other ride. I change my chain once a year because I abuse the daylights out of my drive train but in reality I could probably get away with 2-3 years on a chain.