Also, I feel like vent free might be the way to go to generate max heat- is that true? Being that its outside, air quality is less of a concern.

Can't help you with an insert recommendation, but a masonry firebox would look pretty authentic even with a simple gas log in it. Since it doesn't sound like you intend to burn wood, you could even line the box with an antique brick and refractory mortar to take the heat from the gas log. Sounds like it would match the decor nicely. Nobody wants to sell the vent free units anymore, but since this is outside, you should be fine to use one if you can find it.

The masonry could add to the heat production as it will act as a heat sink during the burn and then radiate residual heat for quite a while once the logs are turned off. Google Rumford design and you will find a nice wide fireplace opening but not a huge firebox that could be constructed without major footings being dug into the ground.