Constructing a heater by applying power to resistors
If your goal is to print things, rather than re-invent the wheel from first principles and repeat many of the learning mistakes made in the reprap community over the last several years, you might as well:


Take one of the already well-proven hot-end heating element &amp; thermistor designs in widespread use and thoroughly published, as their tradeoffs have received substantial practical evaluation.
Take one of the already well-proven control system designs already in widespread use and thoroughly published as their tradeoffs have received substantial practical evaluation.  Most of these are Arduino derivatives, so in cutting them down to only run the heater you should be able to make do with a simple ATmega and the relevant power MOSFET circuit.


What will be left is coming up with a command interface - either you want to tie this into your G-code interpreter for automatic control, or you will want some sort of manual on/off and setpoint control.  For the latter you could also consider some of the published sous-vide type controllers, at least from the user interface perspective,.