The heat mats will come in very handy for germinating the seeds but they won't help you once the seeds sprout as they must be removed from the mats as soon as they break the soil surface. That's when the air temps become important and heat will need to be provided usually at night.

Even this far south of you it would be far too early for me to start anything but lettuces, spinach and other leafy greens, maybe some cabbage in my greenhouse if it wasn't heated at night. Not that it has to be tropical but it does need to be kept above 40-45 degrees. Young seedlings will freeze quite easily.

With night time heat supply - and there are many ways to do this - I can start all of those things plus peppers, tomatoes, most flowers, etc.

So, the real issue is what is your weather like now? Are you still having cold nights or does your temp never fall below 45? If it doesn't then go for it. But if it does, is there a way you can provide some night time heat to the greenhouse?

If not then the best bet would be to start the seeds in your house and only move them to the GH once the temps won't get that cold in it. That is what most people with unheated greenhouses do.

Hope this helps.

Dave