You might want to spend some time planning where plants will go, and how you want to start your plants.
If the only live plants in there at first are going to be a few warm weather starts, you may not need to heat the whole thing. Starting plants respond well to bottom heat (a heat mat). And an inner cover (as per Elliot Coleman: with a few hoops and ag fabric over the starting trays) may be enough. At least until those plants get bigger.

This assumes of course that you are starting a small number of these plants, and not going for 1000's of starts.

Something like this, if you can keep over temps during the day under control, may be a better environment for starting tomatoes et al than an open but heated GH. And of course you'd still have you main heater if a bad cold snap happens.

Thanks Steve,
I had planned on germinating the seeds inside under lights, And then moving them to the greenhouse. I know if i had heat mats I could start right in the GH,but they are not in the budget for this year.

Wow, i am envious of your square footage! I use a natural gas heater in my small palram.
i just finished filling in average temps for my area to a 2015 calendar. Also number of hours daylight and usual first and last frost dates.
yes do plan placement of plants. Yes, do figure out watering, tool, fertilizer etc. Storage wwhere these items will be kept handy.
what type of gh do you have?

I have an "elite" greenhouse With a 10,000 btu heater. I am going to be starting geraniums,lobelia blue and white, petunias in red white and blue. Tomatoes,basil and hopt peppers.

I run a plant sale for Boston Childrens Hospital that runs each fri sat sun in May . I have some great nurseries that donate alot of stuff ,but the GH lets me focus on the items we never have enough of. Google "crazy plant lady" you eill find more about it.

Here is a link that might be useful: elite greenhouse

Did you get the automatic openers for the top vents? That seems like a good idea, provided they work well.

I think cooling and ventilation will be the biggest challenge, especially on a warm, sunny day with no wind. If you are considering any upgrades or modifications, that's where I'd invest the money.

We did get the auto vent openers and DH is home to open doors if need be, I am hoping santa brings us a thermometer we can use with a sensor that either broadcasts to a base in the house or a smart phone