Go for it is my first piece of advice! After a very brief start on a hardtail, I was FS only for years before I was encouraged to get a SS. For me, all the cliches about hardtails and a SS one at that, were true. It really did make me a better rider. The single speed has improved my technique (carrying speed/not braking, picking lines etc) and strength hugely. I remember my first spring ride after a winter on the SS, the White's Level first ascent was a pleasure. I now ride my Inbred more than my Orange 5.
On-one Inbred (26") or the Scandal 29er are all you need, great riding frames and the price is just the icing on the cake. Steel frame are more forgiving on your rearend.
But they may involve some building yourself, handy if you have a load of spares in your garage. And there are many ghetto and expensive solutions to convert existing non SS specific bikes to run with one gear too. If you're looking for a complete new bike though, some common 26" ones are:
Genesis IO
Orange P7
Cotic Simple
Charge Duster
I'll leave it to others to suggest 29ers.