First: Don't ever attempt a project like this that could potential produce fatal voltages if you don't know what you are doing. You obviously do not have the skills or knowledge to attempt a project like this so I suggest you just go buy something that is commercially available.
Second: What you described is not a DC generator, but a permanent magnet synchronous machine. In order to get DC you will need to rectify the output.
Third: You can't model a generators power output with speed alone. Voltage is a function of speed, torque a function of current, and power a function of both. In the case of double-fed machines its also a function of the input electrical work.
What do you plan to do with this power anyway? Charge a car battery?