Hi Minerva - I've put lots of info up on our blog. We try to answer just about every question you may have.
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Remember that a kit with nice high speed such as the Phoenix Racer system will go faster with a higher voltage. You'll also have a more balanced system if you're going to be using the more affordable SLA (sealed lead acid) batteries. My personal system is a 48 V 40 A system. It's really fast (37 mph) - maybe a little too fast for the common user - I just think it's fun for tooling down to the store a few miles and back - I can actually put it on the road and keep up with the cars.
The great thing is that you can really choose a kit for whatever you need. If you're looking at doing around 25 or so, try looking at a Crystalyte Roadrunner kit. They're cheaper and more efficient than the really fast ones and give you a longer distance.
We also have a complete electric bike that will go about 20 mph as well. It's got a range of around 20 - 25 miles depending on how you ride it. Lithium batteries as well.
Congratulations on the consideration of an e-bike!