This is best to use model and sheapest. http://www.reptilica.de/shop/product_info.php/cPath/22_25_38/products_id/35/lang/english/index.htm
8-15 W is good. With 15 W warmer you can warm upp 2-3 box.
If nuc is 5 frame, you can put it in langstroth box with devide wall and the you put heater in empty part. But I think, that if I have bought 5 frame nuc and it has brood, with warmer bees can take whole box into use.
In my experience little nuc developes 3 times faster when you warm it and give pollen patty. But if out temperature is over 68F , hive turns too warm. Night temperature is the point.
I have had 6 W hearter in 2-frame winterball, and bees have survived splended over winter in that way. Normally is no sence to try to keep under 5 frame winterball over winter. Our beewinter is 7 month long.
But as I have said, it is easier if you should have insulated boxes bor nucs. It helps really much at spring.
Still I have heating in my all hives. Half of them have 3 boxes. This week day temperature rises to 68 F, but at night it is near 32F. http://www.wbuf.noaa.gov/tempfc.htm
For little nucs I have bought 6 W heaters. It is good for 2-4 frame.