I would do the walls yourself then, save a decent chunk of money. The floor should be fairly straight forward since it's a new build, the numbers I was hearing last year when I was looking around were coming in about $6-$7 per square foot all in.
If there is a 240v plug in the garage and it's fused for 30A, you'd be fine for installing a heater and running it off that. I'd be a bit surprised if that was done though. If you're working with an attached garage, you can run new wiring and tie into the panel under a homeowner's permit. Very basic work and can easily be done.
I'd call a few companies up and get them to give you a quote for the gas heating. You'll be paying them for pulling the permit, trenching the line out to the garage (unless attached), installing the new piping and tying it into the existing, then installing the heater and ducting. Will be a few thousand most likely.
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