For an "ebike" you should be okay with 450 watts if you really just want "assistance" in getting around. Professional bike racers can't even sustain 450 watts of pedal power over longer distances.
What seems to be happening is that there is an "outlaw element" that is dominating the internet where all these ebikes are being modified so that are way, way, way past legal. (it started with "Big Iron" and has now shifted to RC motors, but the problem is the same)
In America 750 watts is all you get... otherwise you become an outlaw...
So don't be so sold on all this big power talk... 450 watts can still be fun and is legal.
Also, if you are looking for low prices the big power is actually very expensive $$$. So the whole idea of the thread (low price, minimal design) doesn't fit with big power. One of the recent big power modified bikes was sold for something like $5K so it's a totally high priced area to get into.
TNCScooters offer good priced parts on the low end, but it's hard to press those brushed motors to higher levels without them falling apart. If you buy the motor and run them stock and at the rated voltage they will last a long time. I went about 3000 miles on one of the 1000 watt motors before it failed. (running it at it's stock voltage)
On the downside, once those motors fail (it's the brushes and commutators every time) they cannot be fixed so you end up being addicted to buying new motors every time they fail.
For people who just want to get started with ebikes the TNCScooters motors are a good place to start. (buy one of the motors with a built in geardown and you should be able to more or less bolt the motor to the bike and ride)
Note: This #420 chain sprocket is too wide for a normal bicycle chain, but it's the right groove shape... so if you can grind it down to be a little thinner it will work with bicycle chains. Or, you could buy an actual #420 chain and it will work on ordinary bicycle sprockets.
With the built in gear reducer it's best power is at 480 rpm... which means you want to have this 10 tooth sprocket mesh with your front chainring at about 48 teeth in the front. (standard sizing)
You have to decide whether you want one chain or two... two is easier to set up I think... but it's up to you to decide...
----------------------------------
The Cyclone motors are the next step up in price and quality. They are easy to install.
You also might start with a cheap hub motor as an option.
http://conhismotor.com/ProductsList.asp?iPage=1&id=113
...the Crystalyte hub motors are the higher end, but like with everything the price goes up. (From $300 to $600)