Styrene Glazing Wooden Greenhouses
No - a cold frame is generally much smaller and shorter than a greenhouse and won't ever have a heat source.
Not necessarily full sun, but you should consider the movement of the sun and any obstacles like hedgerows and overhanging branches to maximise on natural light.
Yes - seeds like tomato can start in an unheated greenhouse. They may be too cold for some February sowings, though.
For a 6x8 wooden greenhouse, check out our BillyOh Planthouse.