Way too cold. Add 10 degrees F ( 5 C,) just for minimums.

It's a tremendous waste of energy to heat the greenhouse when you should be heating a small electric propagator. You need at least 60F for tomato seeds and much warmer for other things like peppers and aubergines.
Once tomatoes have germinated and you have pricked them on, they will tolerate lower temperatures, once they have grown a bit more.
I'd recommend you look at some of the propagators sold by Two Wests and Elliotts, as they sell heated mats which are fairly cheap, over which you can put a cheap plastic cover. This is easier than soil warming cables.
A good heated propagator is a real investment though and will last for ever (so to speak) and I'm still using the one I bought about twelve years ago.

Hi
I have got a 750 watt oil filled radiator and this is doing the trick the last 2 nights the temp has been around 63 - 65F with the thermostat set about half way at night and switched off during the day as its been around 70 -72 and the best thing its cost me nothing my dad gave it me