None is needed. If you need to set a goal for miles ridden to help for fitness motivation, an odometer is good, if you have a course you ride and frequencly and want to get an idea of how you are doing, speed is important. After that, heart rate and cadence might be helpful.
On my training bike I use speed and cadence, use a HRM and GPS. On my race bike I don't have cadence because my training has taught me the feel and all I use is the GPS and HRM.

So IMHO, start out with a cheap speedometer and if you want more - get more later, your investment will be minimal.

However, if you truely want ot be happy - find out where DF bought his smile-o-meter and get one of those. I've been looking for one for a long time but he won't tell me where he bought his from.