Thanks for all the helpful replies. I purchased a stock Specialized Hardrock 29er. It is a little big for me based strictly on stand over height, but fits me perfectly in every other measurement. I tested a few others, including some smaller bikes, but this one just felt best and was the most fun to ride. My only "upgrade" was the pedals. I also had to get a few basic accessories (helmet, water bottle, stand pump, bike pump, under seat bag, tools, spare tube, and slime in all three tubes).
I've been on a few short rides - pavement, trail, and off-road. I love it, but need to work on my conditioning before I can do any serious riding. In the meantime, I plan to push it a little harder each day, with an occasional rest day as needed. My butt is suffering less than I expected. My legs and lungs, on the other hand, are complaining loudly.