I have used an halogen heater in my greenhouse I assume it would be used in an GH that you intend using but just be cautious with the electric use a mains cut out switch also be aware that halogen bulbs are sensitive to moisture and can blow. The temperature in the greenhouse should be above freezing even with no heat especially if you insulate with bubble wrap in which case the plants should be ok.

thank you i will try it

Tomato,s like a minimum temerature of 50 deg f, and a maximum of 70, any lower than this and you wont get much growth, and any frost at all and you will lose them, I would not plant out until may at the earliest. regards Derekm.