Good websites to learn about bikes are sheldonbrown.com (don't be put off by old-looking site - this was accumulated over many many years by one of the fore-most gurus, now dead) and parktool.com.
The Speed is better than the Roo for you. Forget about a motor, at least for now.
Don't do the $50 bike - it might be such a bad experience to put you of biking completely.