In my experience, and the experience of many people I know, exercise has been less important than diet for weight loss. If you haven't looked seriously at what you eat, and you think you can handle some changes, it might be a good time.
I love my bike computer. LOVE IT. My bike computer is motivating and keeps me honest. When your legs are tired, it's really hard to be objective about how your ride went.
I think my computer was $25 on eBay 4 or so years ago, and those were ~$25 of the best dollars I've ever spent. Performance Bike has the Cat Eye Enduro 8 (like the one I have) for $32 right now. Or for $30 you can get the Astrale, which has cadence (but I don't have that one so I can't vouch for it!).
This might sound a little flaky, but I took a couple of classes recently about health and success in college, and the biggest thing they hit on, repeatedly, was setting strong goals. If you have a goal, you need to make it SMART: Specific, Measurable, Attainable, Realistic and Timely. One of the things that I and people I know tend to do is say "I'm going to lose 5 pounds in a month." Another (better
) way to do this is to say "I'm going to ride my bike 4 days a week for 1 hour each time, and I'm only going to have one Coke this week (on Friday)." Action goals are usually more helpful than "result" goals.
So, point is, if you get a bike computer or heart-rate monitor, it's much easier to say "I'm going to ride intervals of ___mph or ___bpm for __ minutes every __ minutes, twice a week." I was doing that for awhile, staying in a certain BPM range (and not drinking pop), and lost a couple of inches and 8 pounds. (Then I started taking 20 credits of accelerated classes and gained back 5 of the pounds.
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Good luck! Not that you need it. You sound really motivated.