You can get a good "temperature controller" for well under £20 delivered from China via eBay.
See for example the STC-1000 - specimen example
http://www.ebay.co.uk/itm/Mini-Digi...ial_Automation_Control_ET&hash=item3f149ade57
This includes a relay capable of switching 2.5kw.
And it has the smarts to control refrigeration as well (for when you brew lager or make cheese, for example) - refrigerator compressors don't like being switched on/off/on too quickly ...
For use in a freezer, 100 watts of heat should be more than enough.
Excess heating power simply makes the control less stable.
Reptile terrarium heaters should be ideal. Greenhouse ones may be a bit big and/or powerful.
An ex-computer fan, blowing from the heater towards the temperature measurement probe will help make the control more accurate.
A low voltage fan (5 or 12 volts) driven from a transformer fat plug ("wall wort") would be a good safe move. Larger, slower fans are quieter! You don't need or want a tiny "cpu cooler" type.
Try and make all electrical connections outside the freezer chamber - thinking of cleaning, not just spillage.
Safest thing is to connect everything to a single socket strip (so the same earth), and connect the socket strip to the mains through a £5 RCD "safety cut-out" plug.