plex, for MTB, single speed is diffferent than for road, in fact, once you are used to it, you will DOMINATE on the uphills. Your bike will be lighter, and you wont make any mistakes picking the wrong gear, or skipping. you will be very slow on the flats, and i would not recommend it at all for that purpose. As for rigid, I only recommend it from a money saving, i can buy a suspension fork later standpoint. You can afford to get better components for now, and if you are really into it, upgrade later. I wouldn't ride a walmart bike on anything but a paved trail. If you want to go cheap, get a GT or Ironhorse hardtail from your local performance bike shop.
Just for reference, here is my MTB, not rigid, or single speed, but also quite a bit out of rain's current price range and not my first bike.
